When creating a chest cube with /fill ~ ~ ~ ~25 ~25 ~25 minecraft:chest, this cube will only render partially, while creating broken chests and a lot of lag.

Chests aren't an entity (to my knowledge)

The issue could be with transparent blocks, try it with ferns and grass and glass etc

Chests aren't an entity (to my knowledge)

Chests are BlockEntities and rendered the exact same way as Entities.

Oooo, what happens if you try spawing other entities like armor stands?