I've confirmed that at least some ExtendScript works fine in 14.6.0x30.
Can you send a failing snippet?
Ahhh. Yes, ExtendScript from within panels is broken in 14.6.0x37. [Not that it'll help, but ExtendScript executed from within Visual Studio Code or ExtendScript Toolkit continues to work.]
This should be fixed in next beta build; sorry for the inconvenience.
Update: Bug has been patched in today's 18.104.22.168 update. Thank you for fixing this!
Out of curiosity, is there a reason the new UXP stuff keeps messing up third party panels? I would've waited to release that after working out some more of the kinks. I had to rollback 14.5 on my work machine because it seemed to get hung up with that.