lowest val/loss ckpt != highest val/Accuracy
Am using the following callback,
 checkpoint_callback = ModelCheckpoint(monitor='val/loss',
                                        mode='min',
                                        save_last=True,
                                        filename=cfg.CALLBACKS.FILENAME,
                                        auto_insert_metric_name=cfg.CALLBACKS.AUTO_INSERT_METRIC_NAME,
                                        dirpath=LOGGER_DIR)

I am not sure what is going wrong. Am using F.cross_entropy for loss.
Lowest loss (say CE) will not always give you the highest accuracy. (https://kharshit.github.io/blog/2018/12/07/loss-vs-accuracy)
So, always use the accuracy (or desired metric at hand) to select the best model instead of loss.

(This is probably common knowledge but this was news for me. Putting it out there so someone else doesn't bang their head for two days!)