stock & milk: why does reducing the wine help against curdling?

A few days ago I ruined a celeriac soup that asked to cook the celeriac in stock and milk because I made the milk curdle.

I guess I should have brought the stock to a boil before adding the milk. If yes I would like to know why - just to understand. Does the cooking reduce the acid in the stock? (If yes: how? Where does the acid go?)

And is it the same thing with alcohol and milk? E.g. when you make ragu bolognese that also asks for wine and milk is it better to cook the wine until it's evaporated before adding the milk?

To be clear: I'm not looking for a workaround, I want to understand the principle to avoid possible similar mistakes.

Thanks a lot!

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