Does Orisa's Halt! displace a Bastion in Sentry configuration?
Yes, it moves Bastion, but like Zarya's ult, it does not change him out of sentry mode.
Yes, in PTR it does but it's not meant to and it has been logged as a bug
https://us.battle.net/forums/en/overwatch/topic/20753187460
The pull effect from Halt! can unintentionally drag immune heroes, like Bastion in Siege mode, or Mei while she is inside Cryo-Freeze.
I'm assuming Siege mode means Sentry mode and is a misspell on the part of Blizzard.