Can someone knowledgeable please explain what happened here?

I have a r4 spidey supreme and pretty much think I know everything about him. I'm lost as to why he got invisible when I was about to launch my special 2 here.
Any explanations please?
https://youtu.be/Pnmw5Culttc
Any explanations please?

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When you gain a buff while spider totem is active, you get a spider seal charge and getting one of those causes him to enter his Astral form. Mantis gained 2 cruelties from the heavy and the second heavy hit gave her a spider seal buff