Yeah I know what you mean with the past cause present effect. Once mine finds a bug anywhere in the house he will revisit the spot on and off for a month. Once he spotted a mouse which was short lived… he revisited that spot for 3 solid months and just planted himself there.
You added flour(and fat) so this tends to stabilize the milk(it’s referred to as ‘roux’ in French). If the sauce was already thickened the milk will have less chance to curdle. Also if you’re cooking at a low temp it is better and if the milk is fresh and higher in fat it’s less likely to curdle.