Blocks on mob heads not working

When mobs are spawned in with blocks on their heads via /summon, the blocks are enlarged

I summoned a Skeleton with a stained glass head and the block was huge, so I tried some others and they all didn't work.
A skeleton with a fitted "space helmet" looking head

I got the latest snapshot, and I summoned it and it didn't work

I used this command to summon it, sorry if it's a little fast.
/summon Skeleton ~ ~1 ~ {HealthF:60,CustomName:"Runner Skeleton",CustomNameVisible:1,Attributes:[

{Name:generic.movementSpeed,Base:1.4}

,

{Name:generic.maxHealth,Base:60}

,

{Name:generic.knockbackResistance,Base:-1.0}

],Equipment:[

{id:261}

,

{id:317}

,{},{},

{id:95,Damage:0}

]}
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...