I'm not sure that's the case. I assumed the author bought the domain initially for $850, thinking that at renewal it would go down to $12/yr, like most .com, .net, .org, etc. do, but was surprised to find out that the .dev premium domains charge the higher fee every year.

I doubt forum.dev would have ever been $12/yr. Most single word .dev domains are marked as premium and vary from a few hundred to a few thousand dollars annually. I know because I had speak.dev at one point and it was $350/yr and renewal was $350/yr as well.

Now if by some chance they initially paid $12/yr for it and now it's $850, then that is not ok and means that any .dev domain is at risk of dynamic pricing, which I feel like would kill the whole TLD forever for me.