Once you have delivered your assignment, no one should be around to bother you much. There might have been an official policy from the HR, but the Professor's discretion is more absolute.
Each lab has its own culture. In a lab, you are managed by somebody who has not much experience in managing people and time management. So, you have to find your own role within the team and show to the professor that your role is important. But other team members, or even, your supervisor, don't even know what you have been doing in the working hours.
it's sometimes too subjective to your supervisor if you have been performing well or not