Is it the same one every time or is it random? What DWM? Do the two screens use the same type of cable to connect to your GPU? Does power-cycling your black monitor or disconnecting/reconnecting the cable fix the issue?
It could still be a DE config issue since the config can identify screen ID based on some kind of ID given by the hardware (sway can do this for example). What DE are you using and what is the text file config related to screen output?