My first guess would be that your relations are hitting rows that are under row ownership. The data editor won’t fully respect row owners but the front end page will. If the user you are previewing as, does not own the rows you are relating to, then there will not be data to show.
Wow… That was embarrassing… Made a row owner in the morning to check another function and completely forgot about it. Thank you very much, I should spend a bit less time working haha