It looks like this was a combination of dirty Z lead screws and some slicer weirdness. When I started with the generic PLA profile and made appropriate tweaks for the Polymaker stuff, I got bad bed adhesion and shitty prints. Starting with the Bambu PLA basic profile and making the same tweaks, everything comes out great.
(Previously, I’d just been using the Bambu profile unmodified. But a slicer update bumped all the speeds way up, and I was getting a lot of stringing from the Polymaker unless it was absolutely bone dry. So I decided to make a profile for it specifically, and starting from generic was a mistake.)
