I now have the new passport and was as I reported earlier able to go through albeit much slower and with a less responsive machine than I am used to from when I traveled with my old passport (as recent as last month.)

If the theory is true that the system compares biometrics with previous encounters at immigration rather than from just MFA passport data, then maybe it’s simply that travelers on new passports are much more likely to have less data in the system to compare with? I hope that is the case since otherwise 10 years would be a long time to be stuck with a passport with a design flaw. (It would be classic Thailand though if the shiny and pretty new passports are in fact functionally challenged.