Mangrove roots’ rendering distance does not match the logs or leaves. Namely, the logs and leaves render at the same distance as other trees, but roots don’t render until the player is much closer. This causes the trees to appear to be floating until there is a sudden “pop-in” for the bottom half of the tree. This is most visibility on low render distances like those found in realms. To replicate:

1. Locate a Mangrove Swamp Biome.
2. Set render distance to have parity with realms standard. 
3. Move closer and farther away from distant mangrove trees to see the visual issue. 

The mangrove roots seem to almost be treated like entities rather than solid blocks, and thusly are only visible at a much smaller range.