All the new users are getting assigned to the default groups when group Mapping in the plugin is not configured.
To resolve it, you have to configure the group mapping in the plugin, it allows you to map your IDP’s groups to your Atlassian application’s groups. After that, the user will be assigned to the mapped groups at the time of SSO.
For example: if the user belongs to the groups miniOrange-user in miniOrange (let’s assume miniOrange is configured as an IdP) and miniOrange-user is mapped with jira-users in Group Mapping tab of the plugin then that users will be assigned to the jira-users groups at the time of SSO.