How to reproduce:
-Start a Plasma (Wayland) session on a dual monitor setup
-Set the Task manager of the first monitor to Icons Only
-Right click - > COnfigure Icons only task manager
-Enable show only tasks from the current screen.
-Open a window on the first monitor (I opened firefox, but seems to work with every program) 
-Make sure it's shown in the task manager of the first monitor 
-Unplug the second monitor (empty, no windows)
-The program's window is still shown on the first monitor but it doesn't appear in the icons only task manager unless you deactivate "show only tasks from the current screen" even though that task has always been on the current screen (first monitor)