I have been using a Phenidone/AA paper developer off and on for years and I have noticed that it needs a lot of restrainer to keep the whites clean on some papers. I stick with it though because it is easy to make.

I do seem to recall from my research a few years ago that it is better to use an organic restrainer with Phenidone so I use Benzotriazole not KBr for what that is worth to you. I am not an expert though so maybe someone who is can comment on that.