Steps to reproduce the issue
Search forest, or build a small forest
Then, fly above of the trees, and move
Expected result
While flying and moving above of the trees will be not flicker
Actual result
While flying and moving above of the trees will be flicker
If you are talking about the mangrove tree the mangrove roots are treated as block entities and if they leave the simulation distance they will disappear. This is to help performance when players are inside a large mangrove swamp so that the game is playable and doesn't crash or lag