Namharim, Born of Night Harvest bug

Namharim goes into his invulerable state and sometimes will not go into his "spawning adds"-phase for several minutes. Whilst the other 2 harvest bosses does this within 10-15 seconds.

I do not know if it has to do with the new graft hands that mini-stuns or if it is a bug that has been for a while.

This has been happening for at least 4 years now. Not sure why don't they just remove the bloody phase if I have enough damage to kill him, or just fix it.