Hi Morten,
Resources(Personnel numbers) which are having respective business partners with BP role-Employee are only visible in the 'Edit resource' screen.
If you have integrated HR system, then, whether the resource was created in HCM system before integration or after?
If it is before, then you can run the report 'HR_SYNC_PERSON' for specific/range of personnel numbers to create business partners with 'Employee' BP role.
Or otherwise, you can try making a small change to the HR personnel number to create/update the respective BP.
Regards,
Ravi