Dragging a file to a window which is part of a task manager group is broken in several ways: - When dragging the file over the task manager group, you get the pop-up with the individual windows, but hovering the task items doesn't bring the window to the top. - If you now move to a different group, you always get the pop-up of the first group you hovered instead of the one you're currently over.
Confirmed, working on this.
Patch under review at https://phabricator.kde.org/D6231
Committed as https://commits.kde.org/plasma-desktop/84300b516b799fae94fefd7a9a418e5584d37987