This is a problem with all the OB, Mist, and any other sweet wine kits. They market these wines as 28 day kits, when really, due to the extra sugar, they should be marketed as 42 day kits! When you added the F-pack and all the other goodies, you still had a lot of residual yeast in the wine. Sorbate doesn't kill them, but just prevents them from replicating. So, all those live yeast still had a while to munch on all the new sugars you introduced. If your temps were on the low side (69F and under), you actually had a LOT of yeast in suspension, and alive after the primary fermentation.
I recommend that anyone who does mist kits to wait at least 3 more weeks after day 28 to make sure the wine is fully done.