Apparently its possible for planes to spot you over island without you seeing them back.

[Verytis]

Edit: I was in a Lightining.

HE bombers coming. I'm hiding right next to an island

Oh he actually managed to spot me.

I'm running away, thats not good, lets pop smoke.

Smoke right behind me as I slow down, wait why m I still spotted from air. There is smoke between me and those bombers.

Panic accelerate.

Take 2 bomb hits.

Fighters fly over the island.

Wait, I never saw those fighters until now. Wth was that?

[Thyaliad]

Islands are very wonky when it comes to planes. AA can also shoot through islands sometimes.

[Mechfori]

trust the islands to block your sight, but do not trust the islands to block enemies' .. that unfortunately is how this game had made it to be

[Reinhard_of_Avercland]

That depends on the island's height.

If the island is relatively low enough for German & Soviet cruisers to shoot over from, planes can still spot you.

But if the island high enough, it should not be possible.