Cannot set default app for some files (PDF, TXT...) select "always" has not effect, Complete action using keeps popping up
Among the possible apps listed for both .pdf and .txt there was Mylio (a photo manager app). Thinking this might be the root of the issue I uninstalled it and could then open .pdf or.png files straight away (after selecting "Always" once more)...

As for .txt file it still prompts me "complete action with" if I try to open it with Firefox or Amaze text editor (and select Always afterwards). but if I chose Chrome it does not prompt me anymore for some reason...

I don't know if this should be the accepted answer because this is more like a workaround as it does not work with all apps. plus I don't want to uninstall Mylio as I use it regularly.

edit : turns out that if I disable Chrome (could not uninstall it) it then works for .txt file (meaning I can chose any app, select always and it will not ask me "complete action with" again)