Died and lost my things with keep inventory on

This world has existed for 8 months. I play it almost exclusively on an Optifine installation of 1.21.10. I would really prefer to play with keep inventory on, but apparently that setting has somehow been disabled. Keep inventory worked correctly yesterday. I don't want to enable cheats to turn it back on, and my most recent backup is from 3 days ago before I put in a lot of work on things I don't want to redo.
I'm not sure what exactly caused this to happen. The only thing I've done differently since last playing is install Fabric loader, but I created a new game directory for it before I ever booted the game, so how would it have affected my existing worlds? Is there something I can do about this to turn it back on without enabling cheats, so I can still get advancements? I'm upset about losing my 60 enchantment levels and would like to prevent this from happening again.