Programs reappear attached in dock - since Mojave

Hi!

I have a problem I think appeared around the time when I updated my 11" MacBook Air to Mojave. Some programs seems to attach to the right side of the dock permanently once they've been opened.

I therefore have to remove them manually from the dock over and over each day since I don't want them there.

Some of these programs include FaceTime, Adobe Acrobat Reader and Preview. Did I accidentally change some settings, or is this some sort of occurring bug? Any tips? :-)