Yamaha 250 issue

[abl1111]

Got a question for you guys - it's on behalf of my brother who is running (2) yammi 250's with 300 hours and has been having the same issue for 2 years; either engine will suddenly die under way. The priming ball will be totally sucked in. Typically happens after he refuels - can be either engine. Happens randomly. What's he tried;

New fuel lines from tank senders thru to engines including priming balls
All new fuel filters within each engine.
Fuel tank polishing 2x ( after the first cleaning, an engine stalled and the guy came back did it again )

After 15 hours of use, it just happened again..

I think something must be in his tank still, moving about and randomly and temporarily clogging either pick up.

Engines are professionally maintained - the Yamaha guy thinks it's something in tank. With all the baffles etc how can you clean 100% ?

Thoughts ?