I stumbled upon this issue around an hour ago, but actually first entered the cave last night. I saved and quit out of the game because it was getting late, intending to return and finish the area in the morning. I loaded into the game and navigated back to Grime's Grave, went through the whole area, and found the able upon which the tablet is meant to be resting. I was able to collect the opal just fine, but neither the tablet model nor the prompt to collect it appeared. Activating Odin's Sight reveals that the "aura" (sparkly blue thingy) that indicates its presence is there, but the actual tablet is not. I've restarted from a checkpoint, closed and reopened the game, and powercycled my PS4, but nothing works. Hopefully this isn't completely on my end and something that can be patched, because I'm only three tablets away from Excalibur, and I'd hate to be locked out of it. Thanks for anything you can do!!