They syncing back issue is a small piece of fun. Screw it, I’m going to spend a little time today writing a small tool to make that corner of rust not awful, because that corner of rust is awful. I’ll get a long-term fix out today which will make life better going forwards. The reason your workaround didn’t work is because @witty-crayon-22786 (for good reasons) changed the directory you need to delete.

rm -rf ~/.cache/pants/rust{,-toolchain}

should reliably do the job in this case.