I see the OP didn't follow up.  I'm experiencing the same problem.  Attached image from the FMC shows the two states seen on the prog page.  The VNAV wouldn't stay engaged with all the flashing.  For this flight, I was using:

P3Dv4 (4.5.14.34698)
Boeing 757-200 (O.Henry) 757 Captain III Base Pack 1.5
ActiveSky P3D, v7514
Windows 10 Pro, v10.0.19041, Build 19041

Flying at FL350.  The flashing stopped after descending through FL250 or so.  The descent was using vertical speed. 

---