Asked my mom about this a few years back because she had the same rule. Turned out, she knew it was mostly bs, but she didnt want to deal with kids puking up curdled milk because it grossed her out, and apparently we had a harder time puking it up so we were more upset.

And honestly, i dont blame her. Depending on the year, she could have had 5 kids in the house all sick to some degree. If it took telling us milk would make us sicker to help ease that stress, I agree with it.