Office js Web Addin failing to ExpandDL with soap call

Question

I'm currently working with an Outlook Web Add-in with Office.js. In this project, we require sending mail merge emails to handle Distribution Lists and Groups. We use the ExpandDL call and our soap request looks like:

<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <soap:Header>
    <RequestServerVersion xmlns="http://schemas.microsoft.com/exchange/services/2006/types" Version="Exchange2013" soap:mustUnderstand="0" />
  </soap:Header>
  <soap:Body>
    <m:ExpandDL>
      <m:Mailbox>
        <t:EmailAddress>[email protected]</t:EmailAddress>
      </m:Mailbox>
    </m:ExpandDL>
  </soap:Body>
</soap:Envelope>

Our outlook web add-in was working until the recent version of outlook on mac 16.23 (190309). I tested the same thing with an older version older version (16.16) and it worked fine; it returns with the appropriate results for the ExpandDL call.

I traced the code to the file outlook-mac-16.00.js (included from including the office.js library in our web add-in) and ran into this error:

The operation failed.

What's strange is that it was in the middle of formulating the error response but when we add breakpoints to the error and success callbacks, it never hits. The async soap call never returns to execute any callbacks. We had to step into the library to find the above message.

Update: [4/15/2019]

I updated my stable version to 16.23.1 (190326) and we not get another error for the expandDL call:

message: "Internal protocol error: '-2147467259'." name: "AccessRestricted"

The above was extracted from createAsyncResult on line 10914 from outlook-mac-16.00.js when debugging on my current stable version.

Update: [4/18/2019]

I updated to the latest stable version 16.24 (19041401) and this now returns status as "succeeded" but the value of the async result is null. To be sure, I tried the Office.context.mailbox.makeEwsRequestAsync call manually to double check, and the results were the same (null value and "succeeded" status).

I can provide any details on the issue if needed.