Answer
Check whether the workflow's default-data response is including an ETag — Process Enrichment on Default timing normally shouldn't carry one, but if it ends up attached, the page starts treating what should be a brand-new, still-editable record as an existing one with concurrency control applied; explicitly clearing the ETag execution variable (setting it to null) in a script task before the response is returned removes this side effect and restores normal new-record behavior on the page.