Thanks. Unfortunately, that didn't help. Even if I create a new project from scratch, when I record audio it creates new MIDI regions in other tracks seemingly at random, and if those MIDI regions get long enough, Logic deletes the audio at that spot. I don't have any MIDI devices attached, and I've bypassed all control surfaces.
I tried to download an older version (like 10.6.3, which worked fine as a trial version) via the App Store on my old computer, but the latest I can get is 10.4.4 which is glitchy on my Big Sur machine.
Logic is now useless to me. I can't record anything. At this point I have no choice but to switch to another product. I'm looking into Cubase 11 Pro. I would happily pay ten times as much as Logic costs for a product that works. I'd be grateful to hear other suggestions.