Soloing track does not solo track

I can’t figure out why this is happening. When I solo a track, the track is not soloed. I can’t see why this would be happening. I’ve checked all my JACK connections. What could cause soloing a track to not result in hearing only that track?

I’ve tried this on different tracks, with the same result: solo doesn’t actually play only soloed tracks. This is just in this particular session, not in general, so I know something has to be set “wrong” somewhere, but I just can’t figure out what.