$expand returning incomplete "value" in response body

[radanowak]

We have 2 structures - say A and B - where one is a child of the other (B a child of A). Relations are defined in both directions on the same keys. Our problem/question is that the key (and all keys for that matter) of structure B, the child, allows duplicates. When we expand in either direction (As?$expand=REL_Bs -OR- Bs?$expand=REL_A), the response comes back successful (200 OK) but the "value" attribute in the response body comes back incomplete ... just the open square bracket:

"value": [

Is it invalid/unsupported to have a relation to a file without a unique key? Or is this a bug or new scenario?

If I can provide any additional information or clarify anything further, just let me know.

Thanks in advance for any/all assistance.