@Flox Hello again and thanks for the well formed report.
It might be, but it has hopefully been fixed in 3.9.0-13 by @dillif
in the following pr:
which improved this code some.
If you could do the same test with 3.9.1 and confirm one way or the other that would be great. Do please open a GitHub issue if you find otherwise as what I remember as an outstanding issue on 3.9.0, in this area, showed up as a self._validate_input() call that was missing an argument and yours looks different.