Hello,
when placing tiling (concrete, stone brick, ...) you get landfill back. When removing tiling again water is replaced with grass instead of landfill.

Thanks for your bug report,
It's normal that you get landfill back when replacing it with another tile like concrete. That is just how the placeable tile system in factorio works if don't want to get landfill back you disable the "recover landfill" option in the startup mod settings. As to the second bug I couldn't reproduce it when I removed tiling that is placed on water the tile underneath remained water. Is there something I missed?

Have a good day!

Hello again. Sorry for the late response.
Seems like I failed to explain the issue properly. Note the issue is gone now, I cant reproduce it anymore.

What caused the issue was:
1. place landfill mining it now will remove it and give it back (proper behaviour)
2. place tiling (concrete) over landfill you get landfill back, which is fine as well
3. mine tiling, landfill is now replaced by grass (bug)
Note again I cant reproduce this anymore so it is probably fixed. Might have been some conflict with other mods.
Best regards

Hello again. Sorry for the late response.
Seems like I failed to explain the issue properly. Note the issue is gone now, I cant reproduce it anymore.

What caused the issue was:
1. place landfill, mining it now will remove it and give it back (proper behaviour)
2. place tiling (concrete) over landfill you get landfill back, which is fine as well
3. mine tiling, landfill is now replaced by grass (bug)
Note again I cant reproduce this anymore so it is probably fixed. Might have been some conflict with other mods.
Best regards

I've also encountered that bug with grass replacement. I had some landfill in my save before the mod installation, and all that tiles is bugged now.
It doesn't happen for landfill tiles that is placed after the mod installation, though.

Steps to reproduce:
1) disable mod
2) place some landfill and save
3) enable mod
4) place any tiling over that "vanilla" landfill
5) mine that tiling and it's now grass tile instead of water

Also worth mentioning that tiles (stone bricks in my case), which were placed over landfill before the mod installation, properly replacing itself with water after mining.

Thanks for making me aware of this problem,
I was able to reproduce it but sadly I don't know how to fix it. It should work fine if you have just the landfill without any tiles on top though.