thanks, @falkTX! i'll do some more testing to be sure what's going on... i have a knobs'n'pedals box i made which sends CC64, among others. i've used it a bunch with puredata.
i have had some instances in the MOD where a CC doesn't get picked up in learn, and where a previously-used CC doesn't get released when trying to learn a new one. so sometimes it's just necessary to kill the plugin and use a new instance of it. maybe there was an old CC64-learn hanging around... i'll check with a virgin patch and plugins.
of course, a MIDI assignment browser/editor would be super-nice!