Thanks, Hendrik.
At one point, when playing with settings to achieve 4K HDR playback smoothly at 23 fps, I had configured MC to launch mkvs in MPC-BC. I did revert back to MC's "Automatic", however. And now that we have presets, I think the original issue of smooth playback is moot. Interestingly, mkv rips made since I reverted back to "Automatic" play fine e.g. Rampage, which appears earlier in the log. It's almost like some tag got written to those files in the library before a certain timepoint, and now those files always display this errant behaviour.
It's a small, simple library. I think I'll delete the sidecar files, uninstall MC, including registry entries and library files, and then start again.