The bug

Leads do not render when the end positions are perfectly above each other. This rarely happens during gameplay, but can almost be reproduced by using commands.

When a lead is close to vertical in one direction it will render flatly and be hard to see from the sides. I assume that when the lead is perfectly vertical it becomes flat in both directions.

A partial explanation of the bug is that the lead renderer uses a fixed height in the Y axis to achieve a three dimensional look, but that height isn't rotated along with the segments so that it becomes effectively useless

Expected Behavior

Leads in all directions render properly and clearly.

Observed Behavior

Vertical leads do not render properly

Reproduction steps

Execute these commands, ensuring that the pigs don't die (redstone ready superflat should work) to see that leads become almost invisible at certain angles. I have not been able to reproduce the bug where the lead doesn't render at all in vanilla, but it does happen when a modded server sends packets causing perfectly vertical leads

• /summon pig 0 56 0 {NoGravity:1b,UUID:[I;1,1,1,1],NoAI:1b}
• /summon pig 0 60 0.15 {NoGravity:1b,NoAI:1b,Leash: {UUID:[I;1,1,1,1]}