OK, I think I found the issue... It's about dynamic textures and how I thought LL had the old render target + FBO culling issue resolved (scoop: they did not) when I backported the EE renderer to the viewer (and part of the rendering fixes that came with it, in the Windlight renderer).
But what does it got to do with avatar appearance editing, will you ask ?... Good question !... And the answer is: local bakes and texture previews in the appearance editing floater *are* dynamic textures !...
Yeah, not quite obvious, was it ?... In any case, thanks to the narrowing of the bug introduction to the changes that were done between v1.26.24.18 and v1.26.24.19, I could (relatively) quickly find where the problem came from.
I'd gladly accept a confirmation, if you could apply the following patch to either v1.26.24.22 or v1.27.0.2 sources and compile the viewer: