Ad-hoc Group containing list of groups - DO NOT USE

in IBM BPM Routing if we select 'list of users routing assignment where we can provide a string array which should have list of users.

though it is possible to provide the Groups/Roles directly in this list of users using 'Role:<role_name>', it is not recommended to use groups/roles in list of users.

I have tried the same and it worked. 

To be honest,  I was amazed this worked at all. AFAIK routing to a custom list of groups is not supported. As the name says, the routing is "List of users", which is a fairly explicit name. 

One of team in our organization has used this and it was working for more than a year(which is surprising) . Another team has also used same thing last week and all the list of groups/roles are being stuffed in same ad-hoc group and both team started facing issue.

If I had to do this, I would actually create a service that got the list of users on each group (there is an API for that) and then created a list of users by merging them.

Prior to the fix your solution was likely "working" because dynamic groups were never reused, so whatever code path you were leveraging was simly creating a new dynamic group every single time, even if it was an exact copy of another group.