Unfortunately, for some but not all programs (Thunderbird, Signal, Libre Office, QGIS) the minimize and maximize were suddenly missing and appear maximized (not full screen. I noticed it suddenly without having installed anything previously). All other programs appear with minimize and maximize buttons.
Since, I have updated my Pop! OS (to Pop!_OS 22.04 LTS) and tried reinstalling the programs as well as changing the settings in GNOME 3, but without any success. Especially for QGIS without the buttons I cannot move the different window of the program and thus can hardly use it.