Next update: it looks like turning screen lock off will fix the problem, and that even after connecting with lock off you can turn lock back on and it will continue to work ... until the next time you reboot the phone. Also, one post suggested that a May security patch (my phone is version OPW28.1 with an April 1, 2018 security patch, so I assume I have not gotten this patch yet; Google tends to roll out updates slowly at first to make sure nothing blows up on them) fixes it completely.
I don't like the idea of having screen lock off, but I guess that it's not too big a deal for me. I'll try that later on and report. If it works, I'll see about turning it back on after connecting.