The panel has window buttons for each open window, it would be nice to be able to drag them to reorder them in accordance to user preference.
You can change the sorting to "Manually" which then allows dragging the taskbar items. Please check if this resolves your issue.
Glad it has support for reordering, however the object of this bug is to perform drag and drop reordering. I can't even find sorting "manually" plz let me know were it is.
Right-click the Task Manager > Task Manager Settings > General > Sorting combobox.
Goodness lol, KDE is "over customisatable" don't worry its a good thing, a very good thing.