Just wanted to test the new updated 1.17 Pre-Release 3(With the Preview Datapack from the patch notes) and tped to the stronghold and there i found the library where was a part of a Spawner structure in the entrance of the library...
I Put a screenshot of it to the bug report.

Seed: -1451067986286823316
Coordinates: 595 13 -1731

Also i don't know if i needed to post this here or somewhere else because this is no big deal.

EDIT: 
Found more places in this stronghold where caves go through the stronghold.
Coords: 607 10 -1739