Does Trello's REST API enforce a hidden concurrent connection limit per API token that triggers HTTP 429 responses without exposing rate‑limit headers?

When integrating with Trello’s REST API, developers rely on the documented rate‑limit headers (X‑RateLimit‑Limit, X‑RateLimit‑Remaining) to throttle calls and avoid HTTP 429 errors. However, many endpoints omit these headers, and intermittent 429 responses have been observed even when the reported remaining quota appears sufficient. This suggests that Trello may apply an internal, undocumented concurrent‑connection pool limit per API token that is not reflected in the public headers.

The goal is to determine whether such a hidden pool threshold exists, under what request patterns it is triggered, and whether it varies by account type or endpoint.

• Does Trello enforce a hidden concurrent‑connection limit per token that causes 429 responses without updating rate‑limit headers?
• Which request patterns (burst size, concurrency level, endpoint mix) reliably reveal this limit?
• Does the limit differ between free, Business Class, and Enterprise accounts?