I was reluctant to report this bug as it's extremely rare and I have no idea how to reproduce it on demand. However I've noticed it every now and then, and it has been there for quite a while now. When sorting is set to "manually", the task manager will sometimes stop letting you sort items on your own: Trying to drag entries around on the taskbar has no effect, as if the option is set to "alphabetically". Sorting only starts working again after closing certain windows or opening up new ones of a given type. I know that it's independent on which desktop you're looking at and what windows are minimized, but so far I couldn't find a pattern that offers more info.
One little clue I forgot to mention, but I'm very uncertain whether this is related or just a coincidence creating the impression of such: The bug may have to do with window highlighting. I got the impression that when a window went in alert mode (the orange underline) the issue either started or stopped happening.
Is this still happening to you in a recent version of Plasma, like 5.24?
Been a while, trying to remember exactly what this was about myself. I did use to have issues with entries in the task manager freezing into place, something happened and you couldn't drag icons before or after other icons even if manual sorting was enabled, it would typically unfreeze after opening or closing some applications. I think I last saw it an year ago or more, it's possible that it was solved since. If others are sure and no one else is seeing it, it may be resolved by now, if I ever see it again I can reopen or post a new one.
OK, sounds good!