This has been an issue ever since withers were added.

The Invul tag is not capped, and neither is the size decreased based on that tag. When a wither has an Invul value over 870, it starts to get a negative size and turn upside down.

Steps to reproduce:

1. Use the command

   /summon wither ~ ~ ~ {Invul:2000}

2. Observe an upside-down wither that shrinks over time, until it becomes size 0, and then grows again.

Expected behavior:
The wither does not turn upside down (either by having a minimum size, or by capping of the NBT at 860 (before it reaches a size of 0)).