When rendering shapes like a despawn sphere or cylinder together with the biome borders, the individual shape surfaces are only visible from the outside of the shape and all the individual surfaces making up the shape have an additional diagonal line.
Disabling biome borders reverts shape rendering to the more expected state where individual surfaces are visible from both sides. They still seem to be triangulated, though.

Can confirm it's fixed in MiniHud 1.18.1-0.21.3