Hi,
I tested it several times with all the works and she should always get dirty after two days of full work or after a week without a bath.
I'll check it again to see if I can see where the bug you mention is.
Edit:
I found the bug, after working on the brothel and sending her back home she stops getting dirty.
It's already fixed and after the next update she will get dirty again after working. Thanks for the report!