Unfortunately I can't replicate this one on-demand, but on rare occasions, clearing the Windows Notifications (Windows 10) or resuming from Windows Hibernation mode can cause 1-3 Taskbar icons from my group of 4 programs to disappear.
The tabs still appear when I open the group from one of the remaining, non-hidden programs. Un-docking one of the hidden programs from the group does not cause the Taskbar icon to reappear. The only solution is to quit the application and restart it.
Has anyone seen this before?
EDIT: Two additional notes -
1. I am using a saved group.
2. I'm not 100% sure if Hibernation itself is causing it, but I know that it happened the second I cleared notifications from my notification panel on Windows 10.