How to resolve union in Azure APIM synthetic GraphQL

We are kind of experimenting with synthetic GraphQL in Azure APIM, using resolver policies. Up until now everything was working fine, but we want to introduce unions and so far no matter what and how we tried, we couldn’t make the resolver work. Unfortunately we could find no usable documentation or tutorial/posts/etc on the internet (maybe just searching for the wrong stuff…).

If needed, I can provide a few examples and errors we’ve seen so far, but first I wanted to ask in general if anyone has any experience with APIM policies and unions or maybe some examples, that could help us out here.