@JulieC, additionally, we have no idea how long the packet has been setting on the LHBS shelf. Regardless of what else is determined, I suggest you buy small packets that you will use up within 6 to 12 months.
While sorbate appears to be the culprit, let's ensure we all understand the situation and don't miss a different problem. I went to the
Mosto Bello site, but didn't find any details regarding the juices, e.g., what the initial gravity is. Also, it states specifically that the juice will not ferment without added yeast, so the juice is sterilized in some fashion.
While MLF might be the culprit, given that the juice is sterilized, I question if a spontaneous MLF might occur, especially as the juices arrives sulfited. Someone with more experience in that area can chime in.
Do you have a hydrometer? I looked at the equipment package Mosto Bello sells, and it's the first one I've ever seen that doesn't include a hydrometer.
I assume the conditioner is typical, e.g., contains glycerin and oak essence. None I know of contain anything fermentable, so that shouldn't be an issue.
Are you backsweetening, and are you adding K-meta at bottling time?