Recently I've had a number of niggly issues with Betaflight OSD. When I switch to my favourite font (Digital in case you're asking) the save doesn't take on some of my quads. However since I could still add/remove and reposition elements of the OSD I never looked into troubleshooting because it wasn't worth my time.
More recently though when I've switched cameras between quads (NTSC to PAL and vice versa) then I have lost my OSD altogether. The easy fix seemed to be to change the PAL/NTSC in betaflight but this didn't fix the issue. What I didn't realise is that these issue have the same root cause and the answer is surprisingly simple:
Changing between NTSC and PAL here did not save either, this was the cause of me loosing OSD altogether when incorrect