WsAddMemberResults returns code 500 for partial success

I am trying to use WS to replace membership in a group. When the requested subjects have some successful and some failed, WS returns the usual WsAddMemberResults json structure, summarizing the status of each subject. However, the HTTP status code is 500, causing the Retrofit client library I am using to skip parsing the response as a WsAddMemberResults object. Instead I only can get the JSON string which I can't do much with. I think I can add a response interceptor to fake a response of 200 (or maybe 207 Multi-Status would be better?) so that it still will parse the response. But it would be nicer to not have to work around this.

A 500 error is "Internal Server Error" - "The server encountered an unexpected condition that prevented it from fulfilling the request." The request was actually fulfilled, so this error doesn't seem accurate.