Why does the billing role change in Project Worksheet when the assignee is changed?

Question:
A project has been created from a template. When replacing one of the generic assignments with a new assignment specific to this project, the billing role is modified and replaced by another billing role. Why does the not keep in place the existing billing role?

Answer:
There is a default billing role defined in the engagement. Clear the default billing role field in the engagement before updating the project.