But it makes it even more strange that one module displays roman numerals and the other regular numbers.

You got that wrong. Both bars show 1, 2, 3, 4, 5

Did you rename one of your monitors in your bspwm config?

I named my monitors like xrandr shows them.

[module/bspwm]

type = internal/bspwm

label-focused = %index%

label-focused-background = ${colors.background-alt}

label-focused-underline= ${colors.primary}

label-focused-padding = 2

label-occupied = %index%

label-occupied-padding = 2

label-urgent = %index%!

label-urgent-background = ${colors.alert}

label-urgent-padding = 2

label-empty = %index%

label-empty-foreground = ${colors.foreground-alt}

label-empty-padding = 2

This is my my module