Making changes that can potentially alter the CDO or other aspects of a UBlueprintFunctionLibrary can cause BP compilation issues with cryptic messages.
This is caused by the Owning Blueprint (i.e. the Blueprint calling into the Function Library's methods) to attempt to hold onto and use an out of date copy of the Blueprint.
Users have reported similar issues without having to change the class in such ways. It's possible that after the BP is in a broken state it's easier to break again.
The Function with the updated parameters should have a compilation issue in the BP (because the params don't match).
Both of the original Functions will have similar errors:
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45697 in the post.