It currently sits on post and beam. 

Fix the problem.Rip up the old flooring and have the subfloor heavily screwed down into the beams.

Safety hazard.7.4.C loose wire ends need to be in box in storage room8.0 foundation- water intrusion visible signs in crawl space. 8.2- undersized wood beams- need reinforcement10.1.a drywall on wall shows wet stains indicating moisture unit 310.2.A unit 3 living room floors are sloped and sagging.10.4.B sliding door unit 2 10.0.C Drywall on ceiling water stain unit 1 over washer/dryer10.1.C drywall on wall peeling paint at utility closets10.6.C electrical issues10.1.D framing wall leak10.3.D back stairs need to be redone for roper stability11.6.A unit 3 exhaust fan not work11.3.B unit 2 bathtub resurfacing needed11.3.C improper materials used at the unit 1 hot water supply11.5.C defective GFCI outlet unit 16.4.A new sump pump + battery

My general contractor had completed the demo, removed a wall to open of the living area and installed a beam for support.

This installation does not provide proper support for the beams and joists above.

Focus is your friend and choose one thing, have a laser focus until you know it like the back of your hand and move on.

Commercial transactions do not require the seller to disclose faults in the property condition, so over the past few months, I have been taking care of a significant amount of deferred maintenance, including one property that needs the entire floor system replaced ( joists, sill plate, and main beam).

Similar kitchen addition where we put in two metal L beams from party wall to party wall  

Worst case scenario you would need a beam(s) to take the load.

Even if it is load bearing, you can still knock it out by installing a header beam and tucking it up inside the attic.