The resource pack attached offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.
Download resource pack:
The two inner crossed planes which exist inside of mangrove roots do not stretch to the sides of the block. This results in several of the pixels not matchign up correctly between different parts of the block, and also distorts the shapes of the pixels slightly.
Specifically, this is due to the texture planes in question spanning from 0.8 to 15.2, rather than the 0 to 16 that would be expected for this block.
This strongly implies that the mangrove roots model was created by splicing part of the existing "cross" model (with element rotations removed) into what appears to be the powder snow model, as the cross model has planes that span between these two values as to not have their texture planes appear too wide. However, in the mangrove roots block's case, this is pointless and causes avoidable problems.
- Place mangrove roots
- Look very closely at the top and bottom
The inner two planes would align pixel-perfectly with the top and bottom planes of the block.
There is a very significant "desynchronization" between the pixels on the inner cross and the rest of the block, intensifying as the distance from the center increases.
This is a very easy fix - all of the "0.8" values in the model should be changed to "0" and every "15.2" to "16".
Alternatively, the resource pack attached to this ticket can be used instead, which also fixes
The "inverse" of this fix (i.e. changing element dimensions from "0 to 16" to "0.8 to 15.2") can be used to fully fix
MC-236476, a bug I'd strongly recommend looking at again because it's a very, very easy fix.