It used to be for years (or forever) that you wouldn't see the alien dropship if you were not present at the time of its arrival.
Since 18.7.2 arrived, I'm not seeing the dropship during alien raids at all.
Even when I'm the one who disables the cloaking device and then stares for 30 minutes at the spot where the ship is supposed to be - no ship, no beam.
The waves and general spawn fine though.
(this happens in the Montroyal city)
After three city raids in a row with an invisible dropship and beam, both showed up on my last raid yesterday just fine.
Lets just file this under "stuff that happens".