Unity should never really move windows between workspaces unless you tell it to do it explicitly. Now it's enough to put a tiniest part of the window too far (so it's visible on other workspaces) for unity to consider it belonging to more than one workspace. You can switch to this window using alt-tab even if you are not on the workspace where you left the majority of it, and unity is happy to move it around which is confusing.
Unity should never really move windows between workspaces unless you tell it to do it explicitly. Now it's enough to put a tiniest part of the window too far (so it's visible on other workspaces) for unity to consider it belonging to more than one workspace. You can switch to this window using alt-tab even if you are not on the workspace where you left the majority of it, and unity is happy to move it around which is confusing.