That sounds buggy to me....
Only other thing I can think of is the "same" idea, but at the gradebook category level. In your gradebook setup, add a new category -- change the weight of the category to "0" then move your H5P activity into it.
Only other thing I can think of is the "same" idea, but at the gradebook category level. In your gradebook setup, add a new category -- change the weight of the category to "0" then move your H5P activity into it.