mysql> SHOW VARIABLES LIKE '%group_concat%';+----------------------+------------+
| Variable_name        | Value      |
+----------------------+------------+
| group_concat_max_len | 4294967295 |
+----------------------+------------+
1 row in set (0.01 sec)

mysql> SELECT length(GROUP_CONCAT(groupMembers.userId))-length(replace(GROUP_CONCAT(groupMembers.userId),',','')) AS userIds FROM userGroups LEFT OUTER JOIN groupMembers ON userGroups.id = groupMembers.groupId WHERE userGroups.id = 'gid-4802e498-4b29-4eec-bcee-57186b126aa4';
+---------+
| userIds |
+---------+
|    6047 |
+---------+
1 row in set (0.01 sec)

You can try the mysql directly. mysql -uroot -ppassword box. and then (just replace the group id below),

mysql> SELECT id,name,source,GROUP_CONCAT(groupMembers.userId) AS userIds FROM userGroups LEFT OUTER JOIN groupMembers ON userGroups.id = groupMembers.groupId WHERE userGroups.id = 'gid-07e5c661-f43b-45b1-8dc0-8355ca3f05da' GROUP BY userGroups.id
    -> ;
+------------------------------------------+--------+--------+-----------------------------------------------------------------------------------+
| id                                       | name   | source | userIds                                                                           |
+------------------------------------------+--------+--------+-----------------------------------------------------------------------------------+
| gid-07e5c661-f43b-45b1-8dc0-8355ca3f05da | people |        | uid-1b9e3dcd-d91d-462b-8dac-d13f01febb14,uid-ca8ccd80-6cd2-4221-9150-e0b2d69333f4 |
+------------------------------------------+--------+--------+-----------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Can you try editing /etc/mysql/mysql.cnf and adding group_concat_max_len=65536 (just something large) in the mysqld section ? After that systemctl restart mysql

the deletion dialog has the understated member count:

This group still has 1599 member(s). Are you sure this group is not used?

mysql -uroot -ppassword -e 'select count(*) from box.groupMembers WHERE groupId="gid-4802e498-4b29-4eec-bcee-57186b126aa4"'

here's a relevant line for a GET

[no timestamp]  - - [06/Oct/2024:23:59:19 +0000] "GET /api/v1/groups/gid-4802e498-4b29-4eec-bcee-57186b126aa4 HTTP/1.1" 200 36890 "-" "axios/1.7.4"

And the UI does show all group members

In code there is no limit or pagination for the members as such, so the truncating of of the response seems to be somewhere maybe in the reverse proxy or otherwise. Do you see any errors maybe in the nginx reverse proxy logs on the system?