Still no idea what caused this. I tried the steps:
Here for legacy problems and solutions (up to #8)
Here for issues with Nvidia cards
Still wasn't seeing any enemies until they attacked my party first when I reloaded the save.
Since I was at the beginning of the game, however, I started over, and the issue was solved.
It may be that if I had left the level I had loaded into (the caves on the beach) after the fixes I attempted, it would have been solved.
