This is an issue with the attendance Module. The work around it can be to import the attendance for all the employees with part time contract. You can either import the attendances for one day or even a whole year and check in and check out time can be something like 01:00:00 to 01:00:00 and Odoo will then calculate the overtime hours correctly. You can also write a schedule action for this purpose, which will save you the trouble of importing attendances for all the employees with part time contracts.
Thank you for your answer.
This could be a possible workaround.
At the moment i fixed this with creating an attendance from 07:00:00 to 07:00:00 for all the days the employee is absent. It calculates the overtime then correctly, but when i only do this once in a month, the employee isn't aware of his overtime in the meantime. With the amount of workers we have which work part time it's fine. But would be nice when Odoo would be able to calculate the overtime weekly.
Let's see, maybe Odoo v16 brings some improvements.