The sernders MUA is broken and do not set the encoding correctly to ascii text.
GPG suggests to use utf8 encoded in/output. So it is a good guess to just use utf-8 to decode the decrypted content if the Content-Type is Ascii.
This does not break things, because utf-8 is downwards compatible to ascii encoding.
So if the output is really ascii text, nothings changes.

https://bugs.kde.org/show_bug.cgi?id=248058