Terrain flatten doesn't flatten?

Hello everyone,

I see post about this but how do you fix it? The in game terrain editor isn’t that bad, but even the plugins I use for terran I have an issue with flatten. It is like Roblox doesn’t regonize that the terran is still not even.
It will randomly not work on patches at certain spots. Is this a known bug? If so I see post from 2020 about this and on to today.

Edit: I fixed this by using the part to terrain mod like most people did.