Because you use "iced black tea", not if it is ordinary black tea. Because iced black tea is actually lemon black tea with citric acid added. Coconut milk contains creamer.

We used to have a black tea party. Someone made milk tea and someone made lemon black tea. Then, someone had a whim and wanted to make lemon milk tea. As a result, milk is flocculent when it comes into contact with lemon. I asked the biochemistry teacher that it was because the pH value changed (lemon is acidic), which destroyed the potential balance and caused some soluble substances in milk to become insoluble substances.