Strange characters in several widgets since 6.2.1

I get strange characters in QTableView, QDateEdit, etc.

All widgets that probably have some Delegate that draws stuff.

I think it has to do with returning types, other than strings from the model.

E.g.: QDateEdit

But also, when I return from the data() function in the model:

int count = 5;
(...)
if (role == Qt::DisplayRole) {
   return count;
}

I get these strange characters; however, when I return:

if (role == Qt::DisplayRole) {
   return QString::number(count);
}

I get '5'.