Whey powder in pudding QUestion

[arbit]

I make pudding and add whey protein powder to it:
1/3 cup sugar
1 cup coclate whey
2 teaspoon cocoa
2.5 cup milk
Heat till mixture thickens

Turn off heat.
add 2 teaspoons dissolved tapioca starch

Now I heat mixture, it slowly dissolves, and then suddenly it thickens, not a smooth thickening, more like fine particle texture(like indian overcooked spinach), not like smooth toothpaste texture.
I think the whey is causing this.
Am I rite? Can I fix it?