Tasklist: Launch New Instance breakage

Detailed here: https://forum.xfce.org/viewtopic.php?id=16506

Launch new instance doesn't work properly for some apps (thunar, xfce4-terminal). git bisected xfce4-panel back to 4.17.0 but the problem persists, so the bug may exist in one of the supporting libraries. Will start here.

My current system is running all components built directly for the git tree - last build was this morning. Second person with same problem at link above.