Don't really feel like it's a bug... More an oversight.
Lots of things in Gen 7 that should be bank-only for the moment. Two examples from my teambuilding experience
1: Stealth Rocks on Skarmory
2: Rapid Spin on Tentacruel
They're legal egg moves but without bank there's no parents to offer them. Skarmory's only parent who can learn Stealth Rocks is Skarmory, and it can't get it out of thin air, and Tentacruel can only get it from Shellder, Squirtle or Kabuto, only one of which is available, and who also has to get it from Squirtle or Kabuto, who are not currently available.
Understandably it's a HUGE project to go through and find what egg moves on each of the 300~ Pokemon aren't available, hell it seems there's still work going into fixing the availability of some of the hidden abilities like Sheer Force Feraligatr. However, some of them have HUGE potential effects on the current meta, when many of the classic Pokemon can't access some of their classic roles.
EDIT: Slightly ironically, this seems to be effort even Game Freak didn't bother to go through, since you can use Stealth Rock Skarmory in Battle Spot if you hack it in.