This behavior is by-design, according to developers.
IIRC every time you place bricks/concrete there is a chance that underlying grass will be removed, so presumably if you pave and remove the concrete/bricks over and over, you will eventually get rid of all undergrowth.
Thank you very much, i ...

It is normal that grass doesn't disappear when i put the concrete or the stone in the floor? V. 1.2.5

I'm sorry for my bad english, i hope you understand me.