Hi @guwrt
Some possibilities: planes may be multiplayer that leave the session or are slewing?
There is also this bug:
Which could also possibly manifest itself in disappearing aircraft in the VFR map.
I have also seen AI aircraft randomly spawning & de-spawning in the VFR map at various elevations & speeds around my aircraft. I have the base std. edition with no 3rd-party addons. I have no explanation for why this happens - it one of the many glitches in MSFS.