Duolingo app keeps disappearing from favourites

In the last weeks (or even months), I’ve struggled with keeping the Duolingo app on my home screen. When I add it to my favourites, it disappears the next day. Same goes for notification summary. One day I add it to the list of apps for the notification summary feature, the next day it’s not there.

Maybe this could have something to do with the app icon changing multiple times a day? (I do my lessons in the evening, by that time the Duo owl on the icon is furious.) I couldn’t find a way to disable this in the Duolingo app.