Does Vercel serve stale ISR content beyond the revalidation interval under high concurrency?

Goal: Determine the exact conditions under which Vercel's Edge network serves stale Incremental Static Regeneration (ISR) pages while a background revalidation is in progress, especially when request concurrency exceeds normal levels.

Constraints: The platform does not publish the threshold that triggers a switch from stale to fresh responses; observed behavior suggests variation by tier, region, and current load, and the Cache-Control directives may be overridden.

Questions: What concurrent request count causes Vercel to continue serving stale ISR content past the configured revalidation interval? Does this threshold depend on the revalidation interval length or the Vercel region (e.g., iad1 vs fra1)? Are there any configuration options or headers that can force fresh generation instead of stale serving?