Background Process/Batch breaks normal batch_process()'s output.

I have a batch process that works fine, but I want to be able to optionally run it in the background. When I enabled background_batch and background_process (and didn't modify any code or settings), the batch process is now broken.

The progress bar and some text placeholders show up, but the bar itself never fills up, shows percentages, any of my custom $context['message'] stuff etc. However, I can see it's still working because in my Chrome inspector I see all the ajax requests being made by batch, one every second. So it appears almost as if the very presence of background_process makes batch get confused and create a bastard chimera of the two.

Any replies would be much appreciated!
Thanks,
Matt