A leave update changes supply, even when the project plan stays the same. Record the absence against the correct Monday-starting week, then review the person's existing commitments. Reducing a role's demand to hide the absence would change the client's requirement instead of solving the staffing problem.
Part-time working patterns also affect supply. Use the person's actual weekly hours and subtract only the leave for the period being reviewed. The simulator prevents leave exceeding contracted hours, but it still allows you to inspect an inherited overload. After changing capacity, revisit bookings before promising free time to another project. A full leave week can make a previously valid assignment infeasible. When replacing someone, check the replacement's proficiency and grade as well as the open hours; availability alone doesn't qualify a person for a specialist role.