Yeah, it's very likely mod related. Not sure what's causing it though. :/
I noticed you have both Disable Zone Check and Plop the Growables. These mods are incompatible and break when used together. It isn't related to this problem, but you should pick one and remove the other.
You also don't nee Tree Brush when using Extra Landscaping Tools and Forest Brush.
For the problem you're having, I suggest unsubscribing both Extra Landscaping Tools and Surface Painter, then test if the default terraforming works. If the problem is gone, something is messing with ELT. If it's still there, it's a different mod breaking things (upload a new log if that's the case).