This is not an issue, more like a feature request.
In the call tree you can deselect a function call by deactivating its checkbox. As intended, selected children will stay selected. This way only certain actors can be removed from the sequence diagram.
However, I noticed that I often overlook that I have unwanted actors and function calls still selected that blow up my sequence diagram, because I did not realize that certain children of a branch were still selected.
It would be helpful to have the checkbox of a branch show the third state of a checkbox in case any subordinate branches or leaves are still selected.