Hi @JJM ,
Could you tell me how many groups that you own?
This function actually does not have any limit.
If you own more then 500 groups, then that should be delegation limit.
In this situation, you just need to change delegation limit to 2000.

If the problem is not about delegetaion, please check these points:
1)Please check all the groups that you own in azure admin center.
Only the office groups there could be returned by this function.
Other groups like sharepoint group could not be returned by this function.
https://portal.azure.com
2)Please check whether the not display groups's HiddenFromAddressListsEnabled .
If a group's HiddenFromAddressListsEnabled is set to true, this will hide the Group from the organization’s public directory, giving you one more layer of security and control for the most sensitive Groups.
You need to check this in powershell.
Best regards,