I start a new email, and click on the TO button, then add a Group (Distribution list) from my Contact folder.
Send the email.
If I start another new email and start typing the name of the group, Autocomplete shows it and a I can add it to the email.
Close Outlook. Open Outlook.
Start new email, group is no longer available in the Autocomplete list.
I have tried adding the group to the Suggested contact folder. This did not fix the problem.
This only applies to Groups. Autocomplete seems to work fine with singular contacts.