October - last edited November
@Gemmie12I hadn't noticed that one; another thing that's going to bug me. Interestingly, with the walls up, windows and doors do reflect accurately in pool water. Not in mirrors however. Sorry, spoke too soon. Only some first floor windows and doors reflect in pool water and only when you first put wall up.
November - last edited December
@DreamaDoveEA doesn't give ETAs.
It seems like this would be an easy fix: just make mirror reflect what the player sees with appropriate graphics settings. I know it's a minor cosmetic issue compared to crashing, gameplay, and routing problems, but it is rather jarring when you're trying to create something nice and the mirror reflects neon blue outlines of windows and doors instead of, well, the nice decorating you've tried to do. I've been putting curtains everywhere, but it doesn't entirely hide it.
Another thing I've noticed is when doors are opened by sims, the mirror does reflect the part of the door swung open. Go figure.
Edited to add: This really shouldn't affect performance as it was working fine before the update that allowed layering windows and doors. At the very least non-layered windows and doors should reflect properly.
The mirrors are probably reflecting what is really there, in mesh terms. Sims windows don't actually cut the wall mesh, the shader simply interprets the wall cutout as an area not to render. Seems to me the mirror processing may be happening too early? When did this start? Was is round about the time that the lighting was improved to show realistic pools of sunlight inside houses? If so they may have changed the order of processing to make sure the rays fell on everything accurately as a last thing. Just guessing!