Microsoft says that it doesn’t recommend traditional email distribution groups (within Office 365) anymore. Although the company admits that Office 365 Groups can’t handle all the scenarios that distribution groups deal with today, pressure is building to move to Office 365 Groups whenever possible. Among the tools that exist to help make the change is a one-click option in the Exchange Online Administration Center (EAC). However, the option only works for distribution groups that consist of Exchange Online mailboxes. If a group contains any other mail-enabled object such as a public folder or external contact, the EAC option doesn’t work. That’s a problem because many distribution groups contain other types of email-enabled objects, such as other distribution groups, public folders, and mail contacts — or even Office 365 Groups.
When the time came to upgrade a particularly important distribution group, there was nothing to do but perform a manual conversion. And that’s when some problems arose, mostly caused by synchronization delays between Exchange Online and Azure Active Directory. https://www.petri.com/transform-exchange-dl-office-365-groups