If you are creating with the polyline tool and, if at any point in the sequence, you undo a point you laid down, then the Trim Extra Lines feature does not work.
Tried it too and can confirm, it doesn’t work after the undo in both camera and drawing tab.
Confirmed the behavior in my H15 Premium installation (Mac).
Must say this Trim Extra Lines is a great feature.
At least those uncooperative post delete remnants can be dealt with using the legacy tools while they fix this.