Moss growing above waterlogged mangrove roots won't break with the root.

That is because carpets and moss carpets can be placed above ANY block except air, even on water and floating water

Following the above comment, this may qualify as a duplicate of MC-17833, although I'm not sure if this should be kept separate or be resolved as a dupliccate.

Seems like it should be closed as a duplicate, since what's happening is the supporting block goes from being a waterlogged root block to a water block, which never violates that a carpet can be placed on anything.

As yes, this ticket would then definitely be a duplicate of MC-17833. As mentioned above already, carpets can be supported by any block other than air and this is working as intended as stated in MC-17833.