If you create a Summary H5P activity, when a student takes it, the attempt will show up in the teacher's "View all attempts" list (but it shows "0 submitted"). There will be no Score or Date filled in in this list.
If you click on the student's attempts, you will see the attempt listed, with a score, but the Completion and Success are empty.
So the attempt data is getting fed back, but it seems to never "complete" or be considered a submission for some reason, which also means it never gets back to the gradebook.
I confirmed with mod_hvp that the grade does go to the gradebook when the user completes an identical Summary activity.