I am currently using the work around of making the repeats relevant to a check box question and that prevents blank entries being submitted, but it isn't ideal.
I have no solution beyond workarounds. I run into it regularly. They will work fine on the simplest of forms on the initial submission. But when you change page or change position in a repeat they sometimes jump open. When opened to edit they seem particularly bad, 0 length repeats will be opened and blank. I put a required field in one recently thinking that would make sure users saw it and deleted it, somehow I ended up with a blank submitted anyway, required was ignored.
I'm following hoping somebody has a good workaround, and upvoting because IMO the answer is they are buggy and ESRI should work on them.