When you move an item from desktop or from an app (via left click and drag) to a grouped entry in Task Manager, you will see that the textual list is shown over the group, even if "small previews" is selected in Task Manager settings. And when you move your item over that textual list, it gets you to a random window from that group. Normally, what should happen: when you move your mouse pointer over that textual list, the entry below the pointer should be highlighted, and when you release left click it should show the window that corresponds to that highlighted entry.
I am able to reproduce this on git-master as follows: Icons Only Task Manager is present in the panel Opened two KWrite windows, creating a group Dragged a text file from the desktop to the KWrite group in the task manager Confirm that - I saw the text-only list rather than window previews - Dragging the file icon over the group list highlighted the top entry, regardless of which entry was hovered over