If in your location doc the “Recipient name look-up” is set to “Exhaustively check all address books”, this will cause issues when sending emails to groups.
For example, if you set this setting, restart your client, create a new memo, type in any group in your TO/CC/BCC field and hit F9, the group will disappear.
I have the same problem but it’s only happening to certain random groups only. So not all groups are affected.
My “Recipient name lookup” field in location doc is set to “Stop after first match”.
Deleting and recreating the groups with the same names don’t work. Temporary fix is to rename the group with a slightly different name. eg. from “Sales Team” to “Sales Team v1”.
Delete the groups (that disappear when you press F9) from the Recent Contacts view in your local address book. Then try again with the groups and press F9… the groups should stay.
Still a bug but at least a workaround temporarily which doesn’t require you to change the group names.