Operating system: Windows 10 Pro May 2019 Update Build 1903
Graphics card: Sapphire Radeon RX 560 2GB GDDR5 Lite
Broken: 2.80 RC1 (Acquired on 13th of July 2017)
Short description of error
Exporting a mesh with the FBX format, and importing into Substance Painter results in certain faces not appearing at all, or seen from "inside out" (seeing the faces inside the model but not the ones you're supposed to see, outside)
Exact steps for others to reproduce the error
Place a cube, copy it 4 times, extrude them in random ways. Then unite their vertices using "F" to create new faces. Proceed to export via FBX for a simple mesh without any bones, leafs or any of that stuff. Y forward, Z up.
Based on the default startup or an attached .blend file (as simple as possible).