I haven’t had a chance to dig into why this particular thing isn’t working as expected with ObjectPromiseProxy, but in any case there are more Octane-friendly, mixin-less approaches to the problem. I’ve written up a detailed example of migration from ObjectPromiseProxy to an Octane-ready AsyncData type here, and detailed the implementation of that AsyncData type and the corresponding function/helper here. One note: if you adopt the pattern I show there (which we’ll hopefully be open-sourcing a robust implementation of later this month or early next!), you’ll also want to use the @cached decorator on your getter so that you only end up with one API call for any given set of root state no matter how many times you refer to the result of the getter in your template.
Further reading/discussion on @cached: