the issue is old, I have been complaining about it for a year.
Happens when video quality ( signal) goes low , then video link goes back again to normal - the OSD freezes.
resetting power to FC is needed to unfreez OSD, power reset to air unit doesn't unfreeze.
the following is set up:
package-config set msp-osd compress_osd true
package-config apply msp-osd
package-config set msp-osd cache_serial true
package-config apply msp-osd