SameSite is an IETF draft standard designed to provide some protection against cross-site request forgery (CSRF) attacks. Originally drafted in 2016, the draft standard was updated in 2019. The updated standard is not backwards compatible with the previous standard.
Version 80 of the Google Chrome browser has introduced a breaking change in how it treats the SameSite cookie. Other browsers are expected to follow suit, and will experience the issue in future.