More information on this. I instrumented the sources to dump stack traces when these messages are written and arrived at function drm_dp_cec_attach. I further instrumented that to dump the values of the expressions involved in this test:
if (aux->cec.adap->capabilities == cec_caps && aux->cec.adap->available_log_addrs == num_las) {
The result was that every time I have
aux->cec.adap->capabilities == 0b1101111110 cec_caps == 0b0101111110 aux->cec.adap->available_log_addrs == 4 num_las == 4
So the capabilities differ in CEC_CAP_REPLY_VENDOR_ID. If I clear that bit in above test, I am back to normal, getting the "Registered IR keymap" message only once during startup.
Could this be related to commit 613f21505b25a4f43f33de00f11afc059bedde2b?