Hello Henri,
I know you'll hate me for it because this won't be sufficiently qualified (yet -- I am going to try to pinpoint it further in a simple reproducible way first), but I noticed an issue with textures being flipped vertically in a seemingly random manner in 1.26.18 (12) and 1.26.18 (13) -- I am currently cross-checking with 1.26.18 (11), which still seems to be ok.
The impression I get so far is that several factors are in play, namely a (CEF) MOAP display must be active, and surface visibility seems to play a role as well. When I re-checked the release notes, I saw that there were some changes related to texture flipping, so I presume there might be a connection.
I'll add more information as soon as I can. Just wanted to let you know early that something might be afoot.
Love,
Lia
Edit: Added logs of a session that did exhibit the behavior. Started at 17:05 UTC when I opened the door to the neighbouring room (which is on a different parcel), looking in but not yet moving in. Then moving in (which stops the video MOAP in question -- don't mind the other one that's doing cyclic updates, that's just a MOAP text display), which righted everything. Then going back (MOAP restarts), texture flipping on my crossfade picture frames resumes and doesn't stop, even though the door to the next room is closed. Very odd stuff. I hope this helps you make a bit of sense of everything.
You're welcome to visit the back room of my Crossbrook basement which seems to exhibit the issue reliably enough. If you decide to do so it might be best to give me a quick heads-up, the place might be a bit complicated.