I have noticed that Wordpress can be a self-healing thing. Sometimes if you just leave it alone it gets better. This time it didn't get better on its own but I have no idea how I fixed it asde from clearing the cache and turning a few things on and off.

Frustrating when you can't figure out exactly what caused it though. But yeah... there are so many little updates. Auto-updating plug-ins can be helpful but it can also lead to little snafus, I think... glad it healed, at least! 😅

Is it possible a plugin updated itself? If so, you can deactivate it and see if that makes a difference.