When you swap the spawn egg with item not in the 8th spot, the spawn egg acts like the item you switched with.

Expecting it to act like a normal spawn egg.

Steps to reproduce:
1. Put spawn egg into 8th slot
2. Switch slot with another item with action like fire charge or bow. May help with reproducing with lagging machine, and switching it with the 4th slot.
3. Try using the spawn egg.
It will do what you swap the item with. Take all the attributes and the enchantment of what the item had.

One pic with resource pack on and another off.