The <cctype> functions are locale-dependent. Also, they require that the argument is 0..255 or EOF, so it's UB to even feed a char! You need to cast to uchar first! So, basically all our uses of <cctype> functions are UB!
Please check your respective code bases, book patches, if any, onto this ticket, and, once merged, remove your component from the Jira field.
QTBUG-109235 dealt with toupper/tolower, specifically. This ticket is about all the other <cctype> functions, e.g. isprint().