I have several examples where concreep isn't spreading, specifically on lighter areas of the ground, here's an album with some screenshots: https://imgur.com/a/PWJArsz
Are you using another mod that adds tiles? This looks like it could be caused by an alternate landfill mod that's setting hidden_tile to those tiles when you place those patches down.
I haven't placed down any tiles on the ground other than standard landfill, those were generated with the map generator. I am using krastorio 2, and some other QoL mods (but I'm pretty sure none of them alter the terrain)
