Are you sure, Will? :smile: 
I fetched your changes and reverted removal in the `JdbcMessageStore` and this test passed.
You should 
* remove messages from group as it is done in the `AggregatingMessageHandler.afterRelease`
* there is no reason to use `oneOfTwo.getHeaders().getCorrelationId()`, just extract a varialbe `String groupId = "group";`
* you should check if everything is correct:
* after `addMessageToGroup` `store.getMessageGroupCount()` `store.messageGroupSize()`
* the same after `completeGroup`
* than repeate after `removeMessageFromGroup`
* in the repeate the last your line about `isComplete()`

and rename, please, this test method: mention an issue number there.
And there is no reason to have so much comments in this method, especially about `AbstractCorrelatingMessageHandler`.
I'd prefer to have another test, who shows, that this fix works as done for `<aggregator>` and check, that additional message is sended to discard channel.
And run all tests, please, to be sure, that everything OK.
Thanks