I think I’ve been experiencing this for a few weeks now-- where if I used ExtendSrf on a surface with thickness, the direction often flips. It didn’t behave this way before.
As far as I can tell, the surface remembers its initial normal direction, so doing a Flip on it doesn’t change the normal as much as modify it, and it reverts back to the original when you use ExtendSrf. It only seems to happen to “virgin” surfaces (of which I tend to have a lot of when I’m modeling boxy structures and plate stiffeners). Adding a trim to the surface seems to prevent the direction from reverting, oddly.
Example:
Surface has a thickness applied and it’s normal is pointing down (thickness throws up):
If I use ExtendSrf on any edge, the normal direction flips (thickness is now throwing down)
ExtendSrfFlip.3dm (1.7 MB)