MuseScore 3.6.2, problems with 'Save' and 'Save as'

I'm using MuseScore v. 3.6.2 installed from FlatPak on a pc running Mint 20.2 MATE.

Case 1:

I create a new score, do some edits on it and select 'Save' from the file menu. I'm prompted for a file name and enter 'Test'. I don't change the default file type (MuseScore file). MuseScore lets me save a file named 'Test', i.e. with no file name extension (.mscz)

I quit MuseScore and double click on the file 'Test'. With no file name extension the OS doesn't recognize the file as a MuseScore file and lets the standard archive handler open it.

I open MuseScore, activate the File->Open menu entry and navigate to the folder where I stored the file 'Test' created in the previous MuseScore session. MuseScore doesn't see the file until I rename it to 'Test.mscz'.

Case 2:

I let MuseScore create and save a new file 'Test' like in Case 1. I do some changes and activate the 'Save as' menu. I still don't change the default file type (MuseScore file). When prompted for a file name I enter 'Test2'. Now MuseScore will not let me save the file under the name 'Test2' but displays an alert saying 'Cannot determine file type'. I need to add an extension. If I enter '.mscz' as extension I do get the file saved. But if I enter another extension, say '.msc' MuseScore apparently completes the save action. I get no error alert, but no file 'Test2.msc' is actually created.

To me it looks like MuseScore 3.6.2 handles ''Save' and 'Save as' actions in a strange (buggy?) way, at least as installed from FlatPak on my Mint 20.2 pc. Another user, using the same FlatPak installed MuseScore version under Mint 20.1 has experienced the same problems.

Best regards