I too have recently noticed that after retrieving and deleting voicemails, the notification dot on the phone icon does not turn off. This seems to have started a couple updates ago...literally in the past month. Restarting my phone used to clear the message indicator but not anymore. Now my phone constantly indicates I have a voicemail when I don't.
When this started, I also noticed I could no longer press and hold the dot on the phone icon to dial voicemail. Also, doing a down swipe from the main menu and calling my voicemail that way, still does not clear the notification dot or the message in notifications. I have isolated this issue to only occurring when a voicemail is left for me. When I miss a call, the dot appears as it should, and when I open the phone to review the missed call, the dot goes away. The missed call message in notifications goes away also. The notification dot works appropriately with text messages.
I have a Pixel 6 Pro running GrapheneOS Android version 14, updated October 6, 2023. Build number UP1A.231005.007.2023102300