To be accurate, it depends on how the input and output are declared. (cf. Interface group on the left side)
While testing I tried to close and open the editor several times. I didn’t mentioned it because it seemed obvious that the issue was structural as per the invisible (*) non-memory reference. I probably should have.
Keeping a file copy of the interface file, force-deleting the blueprint interface from the project, restoring the file and adding the interface again fixed the issue.
I should probably make a bug report but I keep getting bug report suggestions by email on a safe machine while the UE4 editor is on an isolated windows machine.
(*) unless I was missing something, hence the original question.