For anyone who was having issues with reimporting sprite sheets, I’ve checked in a fix in /EpicGames/UnrealEngine/commit/1310aaef6d7a11b8e3caf9921455a1038576a4e4, but you can also work around issue by opening Textures folder and using right-click reimport on individual texture pages before reimporting sprite sheet asset. seems to solve issue in repro case I was given.
Cheers,