Kerbal Space Program

Please see attached quicksave.

I've just planted a flag. There's no obvious sign of any problem, however the debug log has recorded the following;
[WRN 21:47:46.652] [Flag Module]: WARNING - No Colliders detected in range. Flag may be floating

if I now return to the space center and check the tracking station, my flag has gone.

I was also experiencing bug #497 at this location; this may be related.
I tried a few times to reproduce. I get the floating flag warning about one time in three.

debug log;
[LOG 21:47:46.641] flag(Clone) is in atmosphere. Cannot save.
[WRN 21:47:46.642] [FlightPersistence]: Vessel Flag not saved because it wasn't clear to save: NOT_IN_ATMOSPHERE
[LOG 21:47:46.647] Flight State Captured
[LOG 21:47:46.648] Saving Achievements Tree...
[LOG 21:47:46.649] Saving Achievements Tree...
[WRN 21:47:46.652] [Flag Module]: WARNING - No Colliders detected in range. Flag may be floating
[EXC 21:47:46.654] NullReferenceException: Object reference not set to an instance of an object
...(likely not relevant stuff about pausing/quicksaving/sounds)...
[LOG 21:56:05.245] flag(Clone) is in atmosphere. Cannot save.
[WRN 21:56:05.247] [FlightPersistence]: Vessel I am a flag not saved because it wasn't clear to save: NOT_IN_ATMOSPHERE

I had the same happen with a flag on the Mun. In that case the log was;
[WRN 01:44:47.425] [Flag Module]: WARNING - No Colliders detected in range. Flag may be floating
[EXC 01:44:47.428] NullReferenceException: Object reference not set to an instance of an object
[LOG 01:46:14.252] Active Vessel is about to crash. Cannot save
[LOG 01:46:16.734] [00:02:27]: Jebediah Kerman boarded Command Pod Mk1 on Land on Mun.
[LOG 01:46:16.735] [FLIGHT GLOBALS]: Switching To Vessel Land on Mun ----------------------
...(saving/switching camera/blahblahblah)...
[LOG 01:47:28.019] Landed on the Mun Unloaded
[LOG 01:47:28.020] Packing Landed on the Mun for orbit
[WRN 01:47:28.056] Vessel Landed on the Mun crashed through terrain on the Mun
Again, I didn't find out it was gone until I checked back at the tracking station, but it looks like it was destroyed as soon as I moved out of physics range.

KSP 0.21.1 unmodded