Hello,
there was a mod called Breakable Landfill, which had the same functions and similar bugs to this one.
See https://mods.factorio.com/mod/breakable-landfill/discussion/5ec14c18458184000cb365d0 for more information.

Anyway, here are 2 bugs I encountered with your mod:
1.) You can fill out entire water pools with just 1 landfill and any other tiles, like stone path.
How does it work: First you place landfill, then you place stone path. What happens is you get landfill back into your inventory,
which you can reuse it again, as long as you have any other tiles to place. But the water pool is still filled though.
If you remove the stone path, you get your water pool back.
2.) On the landfill, if you place any structures and remove the landfill again, the structure is deleted without getting it back into your inventory.
This also happens with the stone path or any other tiles you placed on the landfill before (reference to the first bug).

Is it possible to fix those bugs ?
Thank you in advance and have a nice day !

Hello,
first of all thanks for writing a bug report. I have looked into the 2 bugs and sadly none of them are fixable. Especially the fist can't get fixed because of how the mod works. Basically landfill turns into a floor tile like concrete that can only be placed on water. If you now try to replace it it will also act exactly like concrete (or another floor tile) and it will simply get swapped out. So no fix sorry.
The second bug also seems to be unfixable most machines can't be placed in water (because of collision). If the land suddenly gets removed the machine will just disappear instead of being mined by the player and not being mined by the player means you don't get an item.

Thank you for your bug report and have a great day as well !

Update:
You can now configure if landfill is recovered when it is broken/replaced with other tiles.
This setting is off by default.

Thanks for responding.

The creator of the other breakable landfills mod fixed those bugs in his main mod Platforms.
Maybe you can use as a reference to fix it in your mod as well, if it's possible in your version or if you want to.

As an aside: Removing land always has destroyed anything above. I have used a few waterfill mods and if you're not careful you'll even kill yourself by placing water under your character. That's just how the game works.

Yes, but those things can also be dealt with:
To prevent the water-instant-kill, you can use Noxys Swimming
And In the mod Platforms, if you place a building on the platform tiles, it changes the breakable platform tiles beneath the building to unbreakable platform tiles, so you are not able to break those tiles, until you remove that building.

I don't know if it is still relevant but you won't get back double landfill and buildings won't disappear anymore when mining the landfill underneath it in version 1.0.5 and after. So everything in the thread is fixed.