My asumption is that, for some bug, the server is mistaking the plane ID "before" constructing/sending the pos info to some clients. So, CRC is ok, but packet content is wrong.
It his is the case, the client CAN'T trigger a dialog error indicating that an erroneous plane is nearby. It would interfere negatively with you while flying and getting constant error messages.
To elaborate a bit more my assumption lets rewind the tape:
1 - When your client doesnt receive pos updates of a plane in front of you, your client simply keepts it in its last known course and speed, but that plane is still being drawn and visible.
2 - The "invisible" plane possition is being sent correctly to the server by the "invisible" plane client cause while that plane is invisible for you, it may be visible for a country mate flying nearby.
3 - When a plane becomes invisible to you, is because your client receives some data that updates efectively the pos of that plane. If pos is not updated, go to point 1. Remeber that these planes are not natively invisible, they seems to vanish in front of you (for example), that is, they are, somewhat, updated in your client.
Mixing up all these points, my hypotesis is that for some reason the client doesnt render the plane.
Possible causes:
1 - The plane type id is erroneous and the client cant render it.
2 - Erroneous plane coordinates place it well out of range, so it is not drawn in your FE.