I was having the same issue while using the same URL.
Not sure if this will be relevant to you - but I'm hoping it may be to someone else if not as this had me pulling my hair out.
The mailbox I was running this against had been imported into MS365 from Gmail. As such, all of the imported folders had their PR_CONTAINER_CLASS value set to IPF.Imap, once I changed these to IPF.Note using MFCMAPI and relaunched Outlook the API call was returning them.