First, some generic links for future reference:
https://hearthstone.wiki.gg/wiki/Caverns_of_Time
;
https://www.youtube.com/watch?v=NROP88Em5TA
.
Probably a limitation of that functionality, since they are from separate sets after all.
[UPD] This explanation might be related and confirm the assumption above:
The video referenced above also mentions the point.
Are you referring to the Core versions (that Ship’s Chirurgeon in particular), which are illegal in current Twist?
https://hearthstone.wiki.gg/wiki/Ship%27s_Chirurgeon
https://hearthstone.wiki.gg/wiki/Ship%27s_Chirurgeon_(Core)
You sort of have them automatically in Standard.
The Acolyte of Pain is a bit trickier, since it has three versions:
https://hearthstone.wiki.gg/wiki/Acolyte_of_Pain
(From the Legacy set, EXPERT1 in particular, if memory serves)
https://hearthstone.wiki.gg/wiki/Acolyte_of_Pain_(Caverns_of_Time)
https://hearthstone.wiki.gg/wiki/Acolyte_of_Pain_(Core)
You can use either a Legacy or a COT version in Twist and a Core version in Standard. However, after some testing, I realised that the last-named is also usable in Twist for some reason, which may be an oversight or a complication.
Oh, nevermind, I dug up this:
There’s also another link from there: Acolyte of pain legacy cannot be used in twist .
So this particular card might be a painful exception.
[UPD] Or it would seem that some Core cards are usable if they mirror cards from certain sets legal in Twist, but not all of them. For example, Core mirrors of some Legacy Druid cards or Feral Rage can be added to a Twist deck, but a Core version of Raven Idol cannot. This whole shebang appears messy, inconsistent and probably buggy.
Thanks for pointing this out, after some investigation I realised that it’s even more messy than I thought.
Maybe a bug should be filed.
PS Done: Some, but not all, Core versions of cards usable in Twist.
There also used to be this complex of issues: Caverns of time not usable in twist .
What’s this got to do with the TS’s questions?