Setting a plugin module's type to "Editor" causes syntax highlighting in XCode to break for .cpp files in that module. Setting the type back to Runtime fixes the behavior
This has not been reproduced internally.
Attempted to create a plugin and switch Type declaration between Editor and Runtime. No difference in XCode behavior was observed locally.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45084 in the post.