In order for a member of a group to be successfully provisioned to a connected SCIM app, the member (users, in this case) must be successfully provisioned into the app. The six missing member users are likely not provisioned into the SCIM app. They may be present in the app, but if they aren't being provisioned by AAD Provisioning and instead were created manually in the app then AAD Provisioning won't be able to manage their user objects nor their memberships in groups.
Check for provisioning errors on the missing users, or scoping filters. You can also try restarting the provisioning job which will get AAD Provisioning to reevaluate the users and groups and they will likely either be created/updated or generate an error.