Because it does in factorismo factories. It even destroys the hazard marked areas, allowing me to build where I'm not supposed to.

Again Factorissimo is my bane! It seems Factorissimo uses concrete as the "native" tile type rather than as the flooring tile layer, so my mod is not properly detecting what areas to leave alone.

I guess I should limit it to surface 1 anyhow as this could create some weird cases where traffic in one surface can make dirt easier to form on another.

I managed to solve the issue for the most part by concreting over the already concrete floor.
Note: if you do this, do not remove concrete from hazard section while you're standing on it. You will die.

if your only mod, that's adding surfaces, is factorissimo 2 you could edit the control.lua of the mod: change line 12

if ( (p.surface == game.surfaces[1] or not game.active_mods["factorrisimo"]) and (p.walking_state.walking or (p.vehicle and p.vehicle.type == "car" and p.vehicle.speed ~= 0)) ) then

to

if ( p.surface == game.surfaces[1] and (p.walking_state.walking or (p.vehicle and p.vehicle.type == "car" and p.vehicle.speed ~= 0)) ) then

after this change the mod should work like version 0.8.4 (floor tiles in factories don't get destroyed)

Looks like the problem is caused by a typo on line 12 there anyway: it's not "factorrisimo", it's "Factorissimo". Or "Factorissimo2" these days, I think. Note capitals and spelling.

Issue is still happening in the current 0.16 version.