I reproduced it, but I can also tell you that Shotcut does not work that way. There is no function to select part of a clip and remove that part. You can only select part of a clip and add it to the timeline (or playlist). In case you do not like that - oh well - think positive - focus on what you want instead of what you do not want. Buttons on the timeline toolbar only affect the timeline. The '-' button removes the a selected clip on the timline, but you had an empty timeline.