Curious disk issue

I have a problem with the internal SSD in my 6-year-old MacBook Pro. There’s nothing obviously wrong, but Disk Utility shows this, almost instantly

If I run DU from Recovery, it reports (after a long time) that all is fine. If I then restart and run DU, it will report that all is OK on first run, then give this if run again after an hour or two.

If I run First Aid on “Container disk1”, it takes a very long time but completes successfully.

I suppose I could reformat the damn thing and restore from a TM backup, but that would be a PITA. Any ideas?