This may have something to do with the minimum salary for ARC holders (not sure though if TARC falls into this category).
Foreigners holding white-collar ARC need to be paid a minimum salary of NT$47,971, (as far as I know). I bet many employers pay this reluctantly.
It’s common for employers to pay locals between NT$25,000 and NT$35,000 (whenever they can).
Though this doesn’t mean that your sister’s employer isn’t acting in bad faith, since they are going back on their word.
P.S. I work for a Taiwanese company too and am about to get my Taiwan ID, hopefully next week. If they tried to pull this on me, I would just quit.