VM 3.0.18 not assigning new shoppers to default shopper group

[emporos]

Hi there,
I updated a month ago to VM 3.0.18 and discovered recently that new registered users are not assigned automatically to the "default" shopper group. Instead, the field of the shopper group remains empty in the page of the client info. In the list of clients' page however, all users are of shopper group "default".

I figured that it could be an extension that would have caused this so I installed a totally clean copy, but with the same result.
We also created a script with a cron job that changes the "default" client's shopper group to another group after 4 orders and now this is not working anymore.
How could this be solved? we have over 38000 client profiles, so changing them manually is not an option.

Joomla 3.6.5
VM 3.0.18
PHP 7.0.15

Thanks a lot