@oOoCLOUDoOo Flashing from Windows is going to cause serious problems, theres a reason why I design these modified images to be flashed with QFlash directly on system boot or through an EFI shell 
You’re also going to get problems because you are trying to downgrade the firmware manufacturers typically do not allow this because of AGESA changes I override these lockouts allowing for downgrades which is why you must flash these modified images through QFlash (not Winderp version) or through an EFI shell as this will allow for a complete erase and reprogramming of the ROM. Theres also quite a lot of footnotes with GB boards, especially for 3rd gen Zen and 4 DIMM compatibility, specifically in your case; You need at least F40 to run that 3900X, so the F31 version I modified likely won’t do you any good I’d need to check manually if theres any support for 3rd gen Zen with F31 GB simply didn’t bother to mention.
There are a LOT of other footnotes from GB as well you need to be mindful of and if you didn’t do any of these steps you really should completely erase the ROM from an EFI shell to make sure everything “takes” properly. I don’t know why GB made this process for the newer firmwares such a mess, but thats GB for you I suppose.
1. F31 was the last simple firmware image, which is to say no hoops required to jump through before/when updating so consider this your baseline safety net.
2. From F31 and BEFORE updating to F40 you MUST run the EC FW Update Tool B19.0517.1 (or newer) to avoid 4DIMM DDR incompatibility on 3rd Gen Ryzen CPUs (this is probably why you get no POST with moded F31)
3. Now you can safely update to F40
4. Now you can update to F50
If you need to know how to erase and update from an EFI shell I detail how you can do this >>HERE<< under “How To Flash”.