Burden of proof
According to the Ninth United States Circuit of Appeals, an employer who denies an employee reinstatement after they took time off for a authorized reason for medical leave under the Family and Medical Leave Act, has the burden to prove that they had a valid reasons. Historically, courts have gone back and forth. Some have required the employee to prove that the employer did not have reasonable cause to not reinstate them. While other courts have placed the burden of proof on the employer.
If one were to look at the applicable federal statutes, it seems clear that the burden rests with the employer. While the case law is split. Nevertheless, the current authoritative law in Oregon is that the burden is on the employer to provide that they had a reasonable reason to not reinstate the employee after they come back from a legitimate medical leave.