I noticed this last week, but thought it was a temporary glitch, however, it's still persisting so I thougth I'd better launch a report.
I use Firefox on Windows 10. When I go into my set lists (more specifically spare minifigures), I've been selling a couple of things, so certian things I own have changed from '2' to '1' or '1' to '0'. When I change the amount from 2 to 1, it registers correctly and I can also delete the ACM of the double that's gone. However, when changing the amount from 1 to 0, that just doesn't register. Moverover, the number then becomes bold and back to number 1. I can unclick the checkbox obviously, but that does not get rid of the ACM.
Here's a screenshot of one of the bold numbers, I had just changed to 0, but it changed back.
This also happens when you have a list of lots of sets underneath each other.
Hope you can help, let me know if you need more info.