The bad gateway comes from an incorrect ingress values format in the Helm chart. Use the documented ingress values format from the maintainer-linked comment in the issue thread (the reporter confirmed "its working now" after switching to it). The key is matching the chart's expected ingress block shape rather than hand-rolling the Ingress resource.
Source: https://github.com/gravitational/teleport/issues/26130
Source: https://github.com/gravitational/teleport/issues/26130