And, it came back.
I went to the boards manager and reinstalled ESP boards Version 3.0.2 (the most recent) and the problem went away. For about an hour. I installed Version 3.0.0 and the problem went away for a few compiles. And it's back. I reinstalled V3.0.2 and again, my project compiles fine.
I can't just keep reinstalling the ESP boards. What could possibly be happening?
What folder are the boards files installed in? When I reinstall the IDE, is there something else that I should have deleted first?