I finished a game where I summoned 4 Greybough and checked the achievement. It didn’t register any summons of Greybough on the progress tracker.
Did you close the app and go back in? Sometimes it’s a visual bug only and that fixes it.
Doesn’t appear to be a visual bug; I have signed out and back in multiple times and have made no progress on the Greybough quest despite summoning a pretty significant number of them. I’ve definitely surpassed 20, by now.
What game mode did you play?
From 19.0 Patch Notes
- Achievements – Achievements are broken up into categories and subcategories. As you’d expect, the Game Modes and Adventures categories can only be completed within their specified modes. The more general Progression and Gameplay categories can be completed in Ranked, Arena, and Duels modes.
It happened to me too. i played standard. i already summond more than 20 and it did not count
Hi, any progress on this issue? My Greybough count got stuck on number 84 weeks ago. And since reward track is ending soon, I would like to finish cause I believe I summond thousends of them 
Are you still playing ranked or duels, or did you possibly switch to casual?
This was occurring in Standard for me; however, it appears to have been fixed recently. 