I began seeing this error after updating to 3.0.2 from 2.x via the Bitnami chart (helm.sh/chart: discourse-10.0.5). Disabling plugins did not seem to affect the behavior. However, when I added the following Ingress annotations following your advice but adapted for Traefik, the problem appears to be solved for my three replicas.
traefik.ingress.kubernetes.io/service.sticky.cookie: "true"
traefik.ingress.kubernetes.io/service.sticky.cookie.name: "discourse"
traefik.ingress.kubernetes.io/service.sticky.cookie.samesite: "none"
This is the kind of thing that makes me so reluctant to apply apply these major version updates, but we don’t have much of a choice.