Windows expects to have monitors plugged in, or they put the cards into a sleep mode, making them perform poorly and sometimes not show up correctly/at all.  Because RX4x and RX5x GPUs have only digital outputs, they can detect if a monitor is present.  That wasn't really possible with analog connections, hence if DVI-I was on the card, it had to be assumed that a VGA monitor (via adapter) could be plugged in without knowing.

There's no known software fix, unfortunately.  Usually you can get by with one dummy plug per rig, but I've seen cases where it takes more than one card being connected like that.