Code review comment: I've made changes to ensure that we finalize the request after the error response was sent.

Code under review:
protected void respondAsLeader(ChannelHandlerContext ctx, RoutedRequest routedRequest, T gateway) {
		HttpRequest httpRequest = routedRequest.getRequest();
		if (log.isTraceEnabled()) {
			log.trace("Received request " + httpRequest.uri() + '.');
		}

		FileUploads uploadedFiles = null;
		try {
			inFlightRequestTracker.registerRequest();
			if (!(httpRequest instanceof FullHttpRequest)) {
				
				
				log.error("Implementation error: Received a request that wasn't a FullHttpRequest.");
				throw new RestHandlerException("Bad request received.", HttpResponseStatus.BAD_REQUEST);
			}

			final ByteBuf msgContent = ((FullHttpRequest) httpRequest).content();

			uploadedFiles = FileUploadHandler.getMultipartFileUploads(ctx);

			if (!untypedResponseMessageHeaders.acceptsFileUploads() && !uploadedFiles.getUploadedFiles().isEmpty()) {
				throw new RestHandlerException("File uploads not allowed.", HttpResponseStatus.BAD_REQUEST);
			}

			R request;
			if (msgContent.capacity() == 0) {
				try {
					request = MAPPER.readValue("{}", untypedResponseMessageHeaders.getRequestClass());
				} catch (JsonParseException | JsonMappingException je) {
					throw new RestHandlerException("Bad request received. Request did not conform to expected format.", HttpResponseStatus.BAD_REQUEST, je);
				}
			} else {
				try {
					ByteBufInputStream in = new ByteBufInputStream(msgContent);
					request = MAPPER.readValue(in, untypedResponseMessageHeaders.getRequestClass());
				} catch (JsonParseException | JsonMappingException je) {
					throw new RestHandlerException(
						String.format("Request did not match expected format %s.", untypedResponseMessageHeaders.getRequestClass().getSimpleName()),
						HttpResponseStatus.BAD_REQUEST,
						je);
				}
			}

			final HandlerRequest<R, M> handlerRequest;

			try {
				handlerRequest = new HandlerRequest<R, M>(
					request,
					untypedResponseMessageHeaders.getUnresolvedMessageParameters(),
					routedRequest.getRouteResult().pathParams(),
					routedRequest.getRouteResult().queryParams(),
					uploadedFiles.getUploadedFiles());
			} catch (HandlerRequestException hre) {
				log.error("Could not create the handler request.", hre);
				throw new RestHandlerException(
					String.format("Bad request, could not parse parameters: %s", hre.getMessage()),
					HttpResponseStatus.BAD_REQUEST,
					hre);
			}

			log.trace("Starting request processing.");
			CompletableFuture<Void> requestProcessingFuture = respondToRequest(
				ctx,
				httpRequest,
				handlerRequest,
				gateway);

			final FileUploads finalUploadedFiles = uploadedFiles;
			requestProcessingFuture
				.whenComplete((Void ignored, Throwable throwable) -> {
					if (throwable != null) {
						handleException(ExceptionUtils.stripCompletionException(throwable), ctx, httpRequest);
					}
					finalizeRequestProcessing(finalUploadedFiles);
				});
		} catch (Throwable e) {
			handleException(e, ctx, httpRequest);
			finalizeRequestProcessing(uploadedFiles);
		}
	}

Line commented on: finalizeRequestProcessing(finalUploadedFiles);

Revised code:
protected void respondAsLeader(ChannelHandlerContext ctx, RoutedRequest routedRequest, T gateway) {
		HttpRequest httpRequest = routedRequest.getRequest();
		if (log.isTraceEnabled()) {
			log.trace("Received request " + httpRequest.uri() + '.');
		}

		FileUploads uploadedFiles = null;
		try {
			inFlightRequestTracker.registerRequest();
			if (!(httpRequest instanceof FullHttpRequest)) {
				
				
				log.error("Implementation error: Received a request that wasn't a FullHttpRequest.");
				throw new RestHandlerException("Bad request received.", HttpResponseStatus.BAD_REQUEST);
			}

			final ByteBuf msgContent = ((FullHttpRequest) httpRequest).content();

			uploadedFiles = FileUploadHandler.getMultipartFileUploads(ctx);

			if (!untypedResponseMessageHeaders.acceptsFileUploads() && !uploadedFiles.getUploadedFiles().isEmpty()) {
				throw new RestHandlerException("File uploads not allowed.", HttpResponseStatus.BAD_REQUEST);
			}

			R request;
			if (msgContent.capacity() == 0) {
				try {
					request = MAPPER.readValue("{}", untypedResponseMessageHeaders.getRequestClass());
				} catch (JsonParseException | JsonMappingException je) {
					throw new RestHandlerException("Bad request received. Request did not conform to expected format.", HttpResponseStatus.BAD_REQUEST, je);
				}
			} else {
				try {
					ByteBufInputStream in = new ByteBufInputStream(msgContent);
					request = MAPPER.readValue(in, untypedResponseMessageHeaders.getRequestClass());
				} catch (JsonParseException | JsonMappingException je) {
					throw new RestHandlerException(
						String.format("Request did not match expected format %s.", untypedResponseMessageHeaders.getRequestClass().getSimpleName()),
						HttpResponseStatus.BAD_REQUEST,
						je);
				}
			}

			final HandlerRequest<R, M> handlerRequest;

			try {
				handlerRequest = new HandlerRequest<R, M>(
					request,
					untypedResponseMessageHeaders.getUnresolvedMessageParameters(),
					routedRequest.getRouteResult().pathParams(),
					routedRequest.getRouteResult().queryParams(),
					uploadedFiles.getUploadedFiles());
			} catch (HandlerRequestException hre) {
				log.error("Could not create the handler request.", hre);
				throw new RestHandlerException(
					String.format("Bad request, could not parse parameters: %s", hre.getMessage()),
					HttpResponseStatus.BAD_REQUEST,
					hre);
			}

			log.trace("Starting request processing.");
			CompletableFuture<Void> requestProcessingFuture = respondToRequest(
				ctx,
				httpRequest,
				handlerRequest,
				gateway);

			final FileUploads finalUploadedFiles = uploadedFiles;
			requestProcessingFuture
				.whenComplete((Void ignored, Throwable throwable) -> {
					if (throwable != null) {
						handleException(ExceptionUtils.stripCompletionException(throwable), ctx, httpRequest)
							.whenComplete((Void ignored2, Throwable throwable2) -> finalizeRequestProcessing(finalUploadedFiles));
					} else {
						finalizeRequestProcessing(finalUploadedFiles);
					}
				});
		} catch (Throwable e) {
			final FileUploads finalUploadedFiles = uploadedFiles;
			handleException(e, ctx, httpRequest)
				.whenComplete((Void ignored, Throwable throwable) -> finalizeRequestProcessing(finalUploadedFiles));
		}
	}