Same situation with mine at last month, i just get my digi prepaid simcard with a pre-owned number 0138888xxx and getting to insert it into my new 16 pro max.
I switch to postpaid but still showing that red dot badge on the phone icon.
I try to call digi center, the agent only can help me to deactivate the voicemail but it still showing the red dot badge.
Finally, the red dot only can removed by switching from physical sim card to eSim.