The reason is that milk will agglomerate when it meets vitamin C, and apples contain vitamin C, which is not good for health. It's best not to drink together.

The structure of vitamin C is similar to that of glucose, and it is a polyhydroxy compound. The second and third adjacent enol hydroxyl groups in its molecule are easy to dissociate and release H+, so it has the nature of acid, also known as ascorbic acid. Vitamin C has strong reducibility and is easily oxidized to dehydrovitamin C, but its reaction is reversible. Ascorbic acid and dehydroascorbic acid have the same physiological function. However, if dehydroascorbic acid continues to oxidize to diketones, the reaction will be irreversible and lose its physiological function completely.

References:

Baidu encyclopedia-vitamin c