What would make tea astringent for a short time after making it?

Often, when I make tea (English breakfast, most recently Yorkshire Gold), the tea has a rather strong astringent flavor immediately after brewing, but tastes much better after a couple of minutes (at which point the tea has cooled slightly but noticeably). What could cause this?

I usually use two tea bags,* sometimes just one, with filtered tap water boiled in a stovetop kettle. I usually add soy milk and/or sugar or aspartame sweetener. The number of bags and what I add seems to make no difference in this astringent flavor

* I make my tea in a typical American coffee mug, which holds about 350 ml/12 fl. oz, significantly larger, if I understand correctly, than a British teacup. Thus, I am making more tea than the box instructions, calling for one bag, would be written for; using one bag makes rather weak tea.

The core reason your tea initially tastes more astringent and then mellows after a minute or two is how different compounds extract and how your mouth perceives them at different temperatures. Black teas like English Breakfast contain polyphenols (tannins and catechins) that create that dry, astringent sensation by interacting with proteins in your saliva and on your tongue. As one tea site explains, “The astringency in tea primarily comes from tea polyphenols, particularly catechins… [which] explain why astringency plays such a significant role in taste.” (Orientaleaf) Those compounds are more soluble and more perceptible at higher temperature, so right after brewing–when the tea is hottest–you’re tasting the strongest extraction and those drying sensations most sharply. As it cools, the relative perception of harshness drops off; flavour balance shifts and other taste components become easier to sense. Another source notes that steeping conditions affect how much tannin (astringent) gets released: “when the water temperature is too high, it’s not that the tea leaves are being ‘burnt’ … but it is over-extracting tannins.” In short, the temperature-dependent extraction and your sensory perception of polyphenols explain why a very hot, fresh brew can taste more astringent and why it seems smoother after cooling.