Namharim (Yellow Harvest Boss) becomes invisible and invulnerable before transforming into tree

As shown in the screenshot, the yellow harvest boss is in the "tree" phase, but is invisible and cannot be damaged.

Shortly before the boss phased, it did the "turn invisible and fade away" move. It then proceeded to phase and sprout roots, generating monsters without end. However, it is still invisible and does not appear to be taking any damage.

Bug report reference number is 2,318,354,375

It's pure speculation, but I think my Autoexerted Seismic Cry might have contributed? In other Harvests, when Namharim normally goes invisible, I sometimes manage to interrupt its pounce attack. This causes Namharim to walk and chase after me until it gets close enough, where it will resume its pounce attack.

Maybe what happened was Namharim turned invisible/intangible, I interrupted its behavior with Seismic Cry, and then it phased and turned into an invincible tree?