The bug
Entities riding boats, minecarts or other entities are rendered too dark when their hitbox intersects solid blocks (i.e., the ground).
Known instances of this issue
Riding a stranded boat
Riding a minecart on a rail that is placed on solid blocks and is not a slope
Spider jockeys
Video
[media]Related issues
is duplicated by
relates to
Attachments
Comments

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Affects 19w45b. Some entities riding spider jockeys also renders dark
Yes, this affects all cases where an entity rides another entity and its hitbox is intersecting a solid block (usually below it). Closely related to MC-165134.

affects 19w46b