I've seen this as well.
I created a mission with some ground units at an airfield.
It all looked ok when tested in ME, but when in MP the EWR sites (tried both 55G6 and 1L13) didn't show up, but still showed up on RWR's and the F10 map.
I've tried moving them to a different location (Batumi, disused airfield, on top of a mountain, still the same)
I also had the same issue with some watch towers I placed at a FARP.