So I wonder which BIOS did you use to flash your Vega 64 nitro to Vega 56? I read somewhere else that another user killed his gigabyte 64 when attempting to flash 56 BIOS on it, so I guess this is not universal and there is trial and error involved.
It is definitely not universal. You need a bios from the same mfg at least. Get one for your nitro from here: