I understand the frustration. If you have no means of recovering particular groups from the To: line of a previously sent email, it is a very time-consuming process. I'm reasonably satisfied, though, that my current setup, with the "new" outlook.com and
Office 2016 (either the subscription or full version will behave the same), once refreshed in Office Outlook per my post of 9th April above, is stable.
If you are migrated to the new version of outlook.com, and using Office Outlook 2016, you can
Find an old email to the group (should be in your sent items in Office Outlook even if sent from outlook.com)
Select all the recipients (hint - I found this easier if I did a "reply to all" which made it easier CtrlA-CtrlC to copy the whole group)
go into People in Office Outlook,
change the view to List,
click to start a new contact group
Name it
Click to add members
Choose "from Outlook contacts"
Click at the bottom next to where it says "Members" and paste the whole lot in one go
Save and close
and hey presto, you have the new group with the desired members, which should appear somewhere in the list view in bold, with the 2 people icon.
(I practised this a couple of times with just a few of my contacts to make sure it worked)
After a short time, the new group should replicate to your outlook.com and Windows Phone etc.
Hope that helps!