Hi there! I'm currently working through older tickets, and upon testing this issue, I am finding that in 8.3.6, you can still feed Zacian, Zamazenta, and Eternatus dynamax candies. I have found that sometimes, when I give myself one of these three pkmn, the Dynamax Level will already be maxed out and therefore I won't be able to feed them any candies, but most of the time, their d-max level is at 0, and I can feed them.
According to bulbapedia: "Zacian, Zamazenta, and Eternatus are the only Pokémon to be unable to Dynamax." https://bulbapedia.bulbagarden.net/wiki/Dynamax
As such, I will move this issue to internal! Thank you!