🇬🇧 🇩🇪 Headphones plugged in and working, but speakers keep playing sound, too/ Lautsprecher gehen nicht mehr beim Einstecken der Kopfhörer aus

It could be, that this is because of your operating system’s version. Which version do you use? (Go to the Fairphone Updater app, to find out.)

Check out this:

I somehow suspect this to be a bug in Fairphone OS 1.7, since you state, that your phone is quite new. @Zichael_Trouw also reported this issue.