To fix this behavior, you can change the value NULL to '0' in manager_flag column, using pdm_load.
To do it, you can do the following procedure:
IMPORTANT: before continue, please perform a BACKUP of MDB database!
1. Run a pdm_extract on the table:
>pdm_extract -f "select * from Group_Member where manager_flag is NULL" > grpmem.txt
This way, you will extract only the rows where the manager_flag is NULL
2. Notepad the grpmem.txt and replace all the manager_flag value from NULL to 0.
3. Run pdm_load to change the values:
>pdm_load -f grpmem.txt -u
-u is for update only