Imported a bunch of pics earlier today - no issues.
Import a bunch more this afternoon, and now when I click on a photo in Library view - the Quick Develop panel flickers active for a fraction of a second, then appears disabled (all buttons, options greyed out). It does this for all photos in my library.
Resolved it while I was writing this and trying some basic troubleshooting (short of "resetting the preference file", as restarting LR didn't resolve it).
Opened task manager, saw almost a dozen Adobe processes running - killed them all and fired up LR again - WORKS.
I guess the more "appropriate" action would have been a reboot, but I'm lazy.
Leaving this here in case someone else encounters this.
Thanks for the follow up!