let me correct myself. it might be bug on isp side.
for example for polish play.pl i login to account and see they recognize my device model.
then i change imei, imei change with at command always works.
i then relogin to account and mostly i see isp recognizes new device based on new imei.
for some imeis though isp still displays old device, hence i said they see old imei but this could be also bug, perhaps imei is invalid or they did not find device in their database for that specific imei.