Wither moving and shooting before explosion

I encountered this bug when I was showing my little brother what the wither is. The bug applies in the end and overworld.(Not tested in the nether)

What I expected to happen was...:
The wither motionless before explosion, and then moves and shootes

What actually happened was...:
The wither was moving and shooting before the explosion (the spawn explosion that is huge)

Steps to Reproduce:
1.Build the wither (not /summon)