You could cure the problem yourself by pouring a 6" high cement stem wall under the studs from your side of the wall.   

In Mecklenburg County (home to Charlotte) a duplex/triplex/etc. is required to have fire separation that fully separate the units from one another.When I turned a 2-story single family into a duplex a few years ago, we ended up having to pour a cement floor between the spaces.

I also had to let go of a few other helpers along the way and am close to firing the front porch cement guy.

It had an insulated cement floored office building i had planned to turn into a guest room at the time.

I'm sure they work great but that cemented my love for landlord locks after that debacle.  

I considered pouring self-leveling cement and proceed with LVP as planned, but I wonder if that would cause the issue when we fix the foundation and possibly re-joist?

I would still fill those voids underfoot with Super Patch or a similar inexpensive patching compound if you do that.You mention early in the thread about not using "concrete products" on floors that rest on joists...you're talking about cement-based self-levelers.

The stress dynamics of supporting a 2-ton+ vehicle call for special construction if not placed directly on a properly prepared cement or concrete slab.Advice:  address the foundation issues and move on.

So if it’s cheaper maybe just fill it with cement.

So for closing at the walk through the buyer wanted the muddy carpet cleaned...tons of little muddy footprints from the creek, across the cement patio through the carpeted playroom to the tiled bathroom.