Naandi Shraddham is an integral part of Upanayanam and Seemantham. This is an auspicious ritual. In this, prayers are offered to the ancestors and a few Brahmins are rewarded through offerings and this ritual is overseen by the family priest.
They would have said that one should not partake until the completion of the special Naandi Shraddham.
It is unclear how many follow this practice these present days. But one must understand there is a reason why one is asked not to eat before the conclusion of the ritual.