When mob is spawned with mob egg on top of tripwire, dispenser fired arrow doesn't hit them when triggered by tripwire the mob was spawned on

What i expected to happen: mob would spawn and trigger tripwire,firing the dispenser a few ticks after the mob touched the tripwire,damaging the mob.
What happened:the tripwire seemed to fire before the mob touched the tripwire, resulting in Zero damage
To reproduce:
1. Make tripwire, hooked up to a dispenser with arrows
2. Spawn mob in front of dispenser on tripwire.