(Previously each category had its own input field, so the problem arose with the recent change of putting together all the categories in a single input box.)

backspacing should stop at the category border. It looks as if a category entry is treated just the same as a single character. One backspace too many and the previous category will also be deleted. backspace needs at least to pause considerably.

Situation:
Category 1 already entered (x) Category 2 already entered (x) Category 3 just being en

This is especially annoying in combination with the complicated way to add a category. One backspace too many and work is gone.