This problem is reported in the cases when we select "Delete Users from this Domino Directory Immediately", while deleting users. While deleting users, please do not select the option "Delete Users from this Domino Directory Immediately".
If you select Immediate option, the adminp process will start with the "Delete in Access Control list" and person document will be removed by foreground process . If you do not select immediate option, it will create adminp request "Delete person in Domino Directory", which will take care of removing users from Domino Directory including groups.
This issue has been already reported with the SPR # SPR RCDX6C2QNE. Recommendation to not to use immediate option. As removing from group is not a separate adminp request, so there would be no separate log showing the reason for not deleting the user from group. One problem can be not a proper view update, where the username was not fetched to be deleted.