Thanks for the extra info, I tried to replicate this but was unable to reproduce what you described. (I’m thinking this has something to do with the built-in fortnite snap grid, I’ve seen all sorts of insanity (unexpected snap positions and rotations) when dragging around groups of actors that use the FN snap grid)
(Castle still looks normal when in game-mode)
By specific steps I mean something like:
Right Click in Content Browser and create a new Blueprint Class.
Select ‘Building Prop’
etc.
Up to you if you want to put in the work for thorough steps though, there’s a good chance that a similar bug already exists in Epics database.