Payload uses JSON Web Tokens (JWT) for authentication. After log out JWT is not invalidated, which allows an attacker who has stolen or intercepted token to freely reuse it until expiration date (which is by default set to 2 hours, but can be changed).
This issue has been fixed in version 3.44.0 of Payload.
According to WASC, “Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization.”