multiple sound outputs after log-in

When my PC locks the screen after some time and i log back in my sound is muted (was not when I went away from my laptop). If I then turn up the volume again I hear sound playing on my plugged-in headphones and on the onboard laptop speakers. If I reconnect my headphones it goes back to only sound through headphones.

I first noticed the problem about 2-3 weeks ago.

Updated today. Current version: elementary OS 5.1.7 Hera