In a 3/4 measure with alternating eighth notes and eighth rests, I do not seem to be able to manually split the overarching beams (see video below from a new, default project).
This seems buggy to me. I have also attached the Dorico project below.
You can experiment and see what happens. Split Beam assumes that you have a single note selected, and it splits an existing beam at that note. Make Unbeamed removes all beams from the selected notes.