I may have some useful data on the disappearing army problem for you. I had several in my previous fort, and though I deleted the save to load today's release (I'm dumb, sorry!) something happened which was interesting. Two of the disappearing armies were Great Fiend Spiders... and each spider dropped a bag during the few ticks he was on the map. On a related note, I have been attacked by GFS crossbowmen without crossbows repeatedly, which leads me to think they can't hold stuff?
If so, is it possible that snatchers are being chosen as either the leaders or the entire membership of armies? If they arrive as an army they would be instantly revealed when they hit the map, and instantly flee. If GFS can't hold things then their snatchers would appear with bags, drop them, and vanish.
Also, loading up the latest release of FD onto the latest version of DF now, so more feedback on spurring is pending.
Looking forward to your testing!
I have a pet theory that the GCS crossbowmen are so assigned because of archer skill due to their knowing how to shot web. I consider it an easter egg. ^_^
Though they should have custom profession names to mask the obvious mismatch.
The data you report on snatchers causing disappearing sieges squares with my own experiences during testing. However, since implementing snatching behavior is semi-required to guarantee hostile civs, I am not sure if I can currently repair it with my available resources.
Nevertheless, since this bug afflicts the present implementation of vanilla goblins, if this bug is reported and fixed it will also no longer occur in Fortress Defense civs.