This has been happening intermittently as we have grown the number of users in a group.
Using the User Admin to add a user to a group appears to timeout the result is that a large number of member of that group loose their assignment to the group.
Import an SQL file with all the user to group assignments.
('3', 'user', 2)
This works fine so the database is not the error its something within the update code in UAM.
There are only 7 groups used for access to portected pages for each group 'Strata Condo Owners' now one Strata has 503 owners +dozen realtor accounts a small subset has access to another group for pages accessible only by Strata Council. The smaller strata groups have max 100 users and have no problem.
abc abcuser 530
cde cdeuser 90