about the monster Namharim, Born of Night Unique can't can't correctly enter the tree stage

When I play the harvest I have suffered such a bug almost every time fighting with t

Namharim, Born of Night Unique .

I use VIPER STRIKE OF THE MANBA and BINO'S KITCHEN KNIFE,

When the monster was born ，its life disappears about 2/5 rapidly because of the poison，then the monster repeated attacks me and can't be injured for long long time,it even go on more than 1 min.I think there's something wrong so he can't correctly enter the next stage.