In Setup/Misc/Cleanup, try to unmark "download queue" and set "autocleanup every x minutes" at 0 if you didn't do it already.
Edit : thread unlocked. Hecks is right thou, it's not a bug.That thread should maybe be moved to "help" forum.
Edit2 : in NZB#2, import as paused stuff like "small par2 file", "sample", "par2 file if size larger than". Your collections will have paused elements and most of the time they should remain in download queue once downloaded