Opsin parses thioacetic acid as thioacetic O-acid, i.e., the hydroxy moiety is undisturbed. ACD/Labs Name parses thioacetic acid as thioacetic S-acid, i.e. the carbonyl moiety is undisturbed.Wikipedia also favours the S-acid.
Opsin rejects the qualifier for the acid, e.g. thioacetic S-acid, thioacetic O-acid, but accepts it in an ester, e.g. Thioacetic acid S-methyl ester, Thioacetic acid O-methyl ester
I don't know that there is a right or wrong answer, just an ill-conceived overload of thio in this context with no clear guidance from IUPAC that I can find.