Random iPad (new gen 11inch pro) factory reset

Okay, so about 8 months ago my grandfather purchased a brand new iPad Pro, with 128 gigabytes, unlocked/wi-fi. I also had an 11inch pro but mine was a generation older than his, but nearly the exact same, however, his was newer than mine. We got it all set up for him and he used it for 8 months without any problems whatsoever; my grandpa was an artist and was looking to get into digital, thus the reason we purchased the iPad. So we downloaded a program called procreate, a well-known, well-trusted illustration software; he used procreate happily for a long time and even began to excel in it.

Sadly just a few weeks ago he passed away and left his iPad as is, he had changed nothing and had near 15 gigabytes of drawings on the tablet. It was decided my father was to inherit the iPad, which he took and used for less than 2 days and changed absolutely nothing when out of nowhere it just completely and entirely factory reset itself, erasing everything including all his artwork and data on all apps. When I say out of nowhere I mean it, it had been sitting next to my grandpa in the hospital charging the entire time and was removed after he passed when my father took it home looked through photos and drawings, and left it sitting on the coffee table with a nearly full charge. This evening when we got home my dad opened the iPad to be greeted by that screen we all see when we reset our devices; in fear we may have lost everything my father began to sign in immediately using my grandpa's Apple ID and password, the very same one I created for him when he initially bought the iPad. After having done so, he ran through the standard iPad setup where we did not see a recover from backup option.

The iPad logged in and we found everything had been erased. Thank goodness he had his photos backed up on google photos, but everything else was lost. Not to mention the last backup available was from April when we bought the iPad, it was not of any help. All 15 gigs of his art we can never get back is gone, we are a bit enraged and confused. Please if anyone has a solution or just knows why this happened and/or how to prevent it from happening again.