MuseScore 3.6 native dialog problem on Linux Mint 20.1

I never saw this problem with MuseScore 3.5 .
Today i started running MuseScore-3.6.0.423675897-x86_64.AppImage on LinuxMint 20.1 and
File->Save As consistently comes up with a popup that is transparent except for the window frame!
... and thus unusable.
Fortunately i found https://musescore.org/en/node/305684 and https://musescore.org/en/node/304526
which gave me a clue to try Edit -> Preferences -> Advanced and turn off ui/application/useNativeDialogs.
Now a dialog comes up which is quite usable.
It is not however the same (presumeably "native") dialog that worked just fine in MuseScore 3.5
(most recently MuseScore-3.5.2.312125617-x86_64.AppImage).
So i'm guessing something got broken between 3.5 and 3.6 ...
This could be a showstopper to many users.