I think you might be experiencing a more general issue I see from time to time that isn't specific to changing desktops.
From time to time, a tab will just refuse to drop in an existing pane or window. It'll land in a new window (I'm pretty sure I've seen this even when dragging a tab within panes in a single window).
When I click on a tab in the destination window/pane, and then try to drag the tab in, it works fine.
Does this work for you? If so, I'm glad you appear to have stumbled on a way to reliably force the issue to appear--I've never reported it because I've always experienced it somewhat randomly.