I haven't actually noticed a switch from bottom to top. It seems to me that there's a builtin preference for slabs (or maybe all blocks) to be placed in the bottom position.
I'm on android, by the way, so it's not just Windows 10.
Update: As mentioned in the Changelog for the 1.20.70.20 Beta and Preview (in section Features and Bug Fixes > Blocks), the fix provided in the 1.20.60.21 Preview was found to cause a different bug and had to be reverted, so presumably this bug will return in 1.20.70.20 Preview. Mojang will fix it again in an future update.
I haven't actually noticed a switch from bottom to top. It seems to me that there's a builtin preference for slabs (or maybe all blocks) to be placed in the bottom position.
I'm on android, by the way, so it's not just Windows 10.