Just a reminder to people chiming in to this thread - the problem resolved itself, with no known intervention from me, and hasn't recurred yet.
is it just one particular plugin? - yes
does it always change to the same preset? - yes, but different ones for different instances of the synth
does the bug manifest immediately after opening the project, or only after some time/changes ? - it manifests when I pressed play, in that project.
Easy :) Just select "None" in the Patch menu on the midi track. - I don't think I've used that menu for about 10 years. Not that it matters - it seems to have no effect on my synth. It was already set to None, and whatever preset I choose in that menu is ignored, and the one that that already set inside the synth, remains that way.