Found the problem. It’s another unreal bug. It’s the same as: https://answers.unrealengine.com/questions/424938/enum-parameter-with-ufunction.html
Basically Enums & Structs included through the project header will not always (or never) work when used in conjunction with UFUNCTION or UPROPERTY.
The fix is to include a manual-include (so basically uncluding it twice!) for it to work with the unreal headertool when it’s marked as a UFUNCTION or UPROPERTY. Note that Visual Studio does recognize the struct and so does the Unreal Editor but the UnrealHeaderTool does not…