with tooltip icons turned on in task manager (the default) and same apps are grouped, a bunch of icons are displayed across the screen [ see p5-tooltip-firefox.png ] and [ p5-tooltip-xterm.png ].
Notice that there is nothing to distinguish one icon from another.
When there is only one icon, the app's header bar is displayed [ see p5-tooltip-chrome.png ].
If tooltips are turned off and there is app grouping, an iconbox is displayed with small useful icons and the app's header bar contents [ see p5-notooltip-firefox.png ]

Tooltips on are useless when apps are group, but useful for single instance app.
Tooltips off are useful when apps are in a group, but useless for single instance app.

Reproducible: Always

Steps to Reproduce:
with tooltips on, hover over grouped app with two or more windows, grouped apps are not distinguishable from the icons
with tooltips off, clicking on grouped app is distinguishable, but other single window apps are not identifiable.