Forge version:
Twilight Forest version: 128
(did not crash)

Steps to reproduce:

1. Complete the mod up to the phantom tombs.
2. Wander through the maze
3. Look for a double-decker goblin

What I expected to happen:
I wanted to find a cool mob which was actually 2 different mobs.
What happened instead:
I found the upper knight, but not the lower.

Oops! Called it the wrong thing XD

Looks to be related to https://bugs.mojang.com/browse/MC-103516

the spawn egg and /summon command all work just fine. Spawning naturally is the issue as it fails the isNotColliding check in WorldEntitySpawner

there's a forge patch in for this, not much we can do here on our own, the bug affects all naturally spawning stacked entities

Linking this here and also gonna take the milestone off, don't think this issue needs to be in it tbh
MinecraftForge/MinecraftForge#4429

Since there is no problem on our end, this issue will effectively be patched when the version of Forge with the Patch will be released. Closing issue