Update: I was able to reproduce the issue. In my case, I also need to turn off the screen (i.e. put the phone into sleep mode). When I unlock it again, and hop back into the game, I can’t save anymore. My findings so far:
1. sd card access breaks when I go to phone desktop AND put my phone into sleep mode.
2. one of these two actions alternativey does NOT break sd card access. So if I put my phone to sleep while running MagicDosbox as foreground app does not affect sd card write access
3. this is a temporary issue, restricted to the currently running game. So when this happens, and I exit the game back to the Magic Dosbox app folder, then start the game again immediately, sd card access is fine again.