I think I may have stumbled on the issue through some other similar looking issues. Issue #1754 eventually pointed to a post in this forum from November '17: New Dell PC with Windows 10 Home - MP Doesn’t see the serial ports and crashes.
That post links to the CP210x drivers page, but specifically circles a driver download that is one version older than what I’ve installed on my two newest systems - in fact, the latest driver released in November 2017, which came after any previous computer I’ve installed that driver on. I won’t be able to test it until tomorrow, but maybe there’s something odd in that latest “Windows 10 Universal” driver that doesn’t agree with MP? I’ll reply to this thread after I try it and see if that’s the fix.