1. project uses deferred BUT when you switch to isometric - deferred cannot be used with ortho camera so editor is switched to forward
2. fog settings are used ONLY in forward (for deferred you need fog-as-post-effect)
3. when you change scene camera perspective<->ortho camera distance changes too (and considerably i must add) so fog kicks in much earlier

Though, i must add, even if fog is switched to linear it is still way more abrupt change than in perspective

it is not a regression per se as you can see in screenshots that there were parts "fully in fog" before - it is just start way earlier now