If above workflow was run manually using workflow_dispatch event, then github.event has different payload than the ArgoCD Webhook expecting. You can check argocd server logs for more info about this webhook event.

ArgoCD Webhook expect a push event
Source code - https://github.com/argoproj/argo-cd/blob/master/util/webhook/webhook.go#L159
Push event payload - https://docs.github.com/en/webhooks/webhook-events-and-payloads#push

Where as workflow_dispatch event has different payload - https://docs.github.com/en/webhooks/webhook-events-and-payloads#workflow_dispatch