some registered customers get wrong customer group

[frontDev]

In some cases registered customer gets Guest group (id: 2) and some get Customer group (id: 3), but default group_id is set to 3.

Only way/location to register is regular register page (no register option after checkout). I cannot figure out why some customers get wrong group (2). I've been unsuccessful to reproduce the problem locally... What could be the problem?

Prestashop 8.0.1

Edited September 18 by frontDev (see edit history)

[Knowband Plugins]

To troubleshoot the issue Look at the customer database (ps_customer) and see if any of the incorrectly assigned users initially registered as guests (check the is_guest field, which should be 0 for registered users).
If users initially registered as guests, investigate if there's a guest checkout feature being triggered unintentionally, or if these customers had interactions with the system before fully registering

You may need to check the overrides as well
Check the override folder for any customizations to the Customer class or registration controller (AuthController).
Review any customizations for inconsistencies in group assignment logic.