This is the expected behavior. ID service (the calls to dpm.demdex.net) will only fire on the first page of the first visit in order to get an ID. Once that ID is stored in the AMCV cookie, there's no need to keep using up network calls in order to get Experience Cloud IDs. The same is true for the ID syncs with our common data partners (Liveramp, Google, The Trade Desk, and so forth). All of those ID syncs only fire on the first page of the first visit and then they'll do a refresh about once every 14 days/2 weeks, just to make sure we keep fresh IDs for our data partners.
The only exception to this are ID syncs that you may be doing as part of Authentication. So as your users authenticate to your mobile or web properties and you perform ID syncs with your own customer IDs, then you'll see those fire upon authentication. You should also so your IDs in all of your Analytics, Target, (and AAM if using stand alone DIL) calls on all subsequent pages until the user logs out. But yeah, standard ID service and partner ID syncs only fire the one time and then the occasional two-week refresh rate afterwards.
