did you buy used? i have Radeon RX580 bought used that afaik was used as a mining slave in China, that exhibits the same problem. it has 2x HDMI, 2x DP, 1x DVI output... sometime some of HDMI and DP dont output any signal, sometime they only show display when entering Windows (hi resolution hi color display) but blank during boot time (lo resolution mode), reading the net sometime its monitor problem, ie auto detect between HDMI/VGA may missed, so recommended to use manual HDMI mode. i tried set monitor to manual mode, but in my case, problem still the same. so the workaround now, i use the DVI output with DVI to HDMI adapter, to my monitor's HDMI input, which is so far stable on both boot/bios and in Windows mode. ymmv.