Oh, that explains why I encountered no bug. I had a raid boss spawn.
Well hey, you can only spawn 1 SG at a time, right? That means we could run 1-3 until we get the 12 stam SG, and then complete ?-8 in under 4 minutes to guarantee that it won't be bugged... right?
Edit: Nevermind, apparently you can spawn multiple Secret Gardens.