Wow that was fast, not only you replicate the issue within 5 minutes, but also during lunch break.
Anyways, just additional information. It seems if I create another editing page and add the 'product_picture' data from that page (meaning I separate the two data entry), then the data seems to be correctly inputted. So I would guess that the error is in the form page. It seems that if we have two entry fields, both with 'files' type then the 'add' button would somehow fail to enter the data to the correct relation.
Hope you guys find the bug and fix it.
But again, thank you for this wonderful platform. I appreciate so much what you guys do.