This may be a regression with the new API, but I don't think you can handle non #inventory based model ids through resolveModel methods.

This is a problem in my case, where I wish to register dynamic block models created by the user.

ModelResolver never supported ModelIdentifier (Identifier + variant) models as that is not its goal (use BlockStateResolver or ModelModifier.OnLoad instead), so I assume you mean that models that are referenced in blockstate files are not received by ModelResolvers. The code should support this; can you provide the code that stopped working?

Sorry for the long wait, I've noticed that my code for this wasn't even working in the first place, and I've since realised I was doing it wrong. I'll be marking this as invalid, and going with the intended methodology.