According to the PMBOK Guide, an issue log is a project document that records problems in a project and how they are resolved.
A resource management plan is a component of the project management plan that describes how resources will be estimated, acquired, developed, managed, and controlled.
When a team member leaves the company, the project manager should update the issue log with the details of the problem and the impact on the project.
Then, the project manager should refer to the resource management plan to find out how to deal with the resource gap, such as reassigning tasks, hiring a new team member, or requesting additional resources from the sponsor or other stakeholders.
References:
* PMBOK Guide, 6th edition, pages 89-90, 318-319, 374-375
* Agile Practice Guide, pages 32-33, 77-78