As a little heads-up: lDosbox suffers from the exact same issue. So it’s not related to Magic Dosobox in particular, but seems to be a problem with the 64bit (dynrec?) core instead.
Older Dosbox ports, as well as the 32bit build of Magic Dosbox, are working, but there’s a huge issue with that: Google will drop supprt for all 32bit apps in 2021, which means developers won’t be able to upload new 32bit apps, or updates to existing ones. So the move to 64bit is kinda mandatory. ☹️