Mobs lose interest in killing the Wither boss

When spawning a wither in a bedrock cage, any mob that will attack the wither, start out attacking as they should. Then sometimes when the wither reach 1/5th health left or lower, they stop and completly ignor him.

I have tried with:
Iron Golom
Vindicator named Johnny

Same thing happens with both, it does not happen every time, but most times.