Grainy/gritty sauce

The user has been making a tomato sauce for years, similar to a viral "vodka pasta sauce," but recently it has started to "curdle" or appear grainy with tiny white-ish bits. This issue began in the last few months. Previously, the user used unsweetened almond milk in their sauce. More recently, they have reintroduced dairy and now use whole milk. The recipe typically calls for heavy cream, but the user finds it too rich. The grainy texture appears after adding milk. The sauce-making process involves sautéing onions and garlic in olive oil, adding chili flakes, cooking off tomato purée, and then slowly adding milk. The user is perplexed as this problem is new and they have never encountered it before.