Why are my aerial photos removing the mountains in the background?

That's Sky Replacement interpreting distant mountains as sky and replacing them. It happens on aerials where mountain tones blend with the sky.

Fix for the current shoot

Go to autohdr.com/account → Style Settings, toggle Sky Replacement off, then re-upload. Re-runs are free since you only pay when you download — the reprocessed shoot will have the real mountains intact.

Going forward

You can deselect Sky Replacement when uploading aerial shoots specifically, and leave it on for everything else. Or keep it off globally if you mostly shoot aerials.