I guess I'm late to this discussion, but I have another thought.

This past fall, Ihad a batch of syrah which was doing the same things
as your Cab mix. Ultimately, I innoculated for MLF, and then
re-innoculated after several months. Still, I was getting persistent
minor bubbling. Finally, after fooling with this wine for about 7-8
months, I just tried degassing it with a stirring wand on a drill.
After LOTS of agitation, the bubbling stopped, and hasn't
re-startedafter another 2-3 months. It tastes "sweet", but the R.S. is
virtually zero.

So, I wonder if the sweetness is just the flavor of your grapes, and
the bubbles are retained CO2.

Lee