Certain weapons create ice when you hit a mob with them. If the mob is in a partial block like a slab, that block will be replaced, even if it's claimed with FLan. This is both easy to exploit and very annoying in player-made builds.

If the ice melts, it will create water. This water gets cleared when the ice gets cleared, luckily, but it still breaks blocks like torches and redstone.

Livyatan & Frostfall will no longer break half blocks in version 1.46.1 (pending release on CF).
The ice blocks can still melt however. I'd recommend disabling these two weapons from spawning if this is causing issues.

Converting to Frosted Ice rather than Ice and making sure that blocks only replace Air would probably be a good starting point for this issue, @Sweenus