Looks like a bug, but I would like someone to confirm.

Whenever I manually group a number of windows, belonging to the same app, the grouping is undone and all individual windows are in the taskbar again after I switch desktops and return.

To reproduce:
- set taskbar grouping settings to 'Manual'
- set taskbar to only show windows of current desktop
- open a number of different windows belonging to same app (in my case: Skype)
- group them manually (hold Alt key and drag and drop taskbar entries)
- switch to another virtual desktop (both with effects enabled or disabled)
- switch back to original desktop
- grouping has disappeared.

I looked for this bug but could not find it, so it may not exist or not have been reported yet.

Confirmed. See https://bugs.kde.org/show_bug.cgi?id=200255

Thanks. Somehow my query skills seem to be deficient