In summary, it seems like some of the patterns on the walls of the woodland mansion seem to be incorrectly rotated. All the logs are facing up with no regard for the decoration, which wasn't the case before.
This affects all mansions, but using the "Spooky Mansion" template when creating a world makes for easier viewing.
Separated the entrance slabs into a separate report (MCPE-169038), as that happens in the regular version too and not just the preview.