Flex Consumption intermittent slow host initialization

#### Investigative information

- Timestamp: 2025-12-16T05:52:49.3675888Z
- Function App version:
- Function App name:
- Function name(s) (as appropriate):
- Invocation ID: 2721b80b-8fe3-41fb-a917-84fc63bdfed3
- ec6fc964-7894-4491-babe-669a14946f84@2025-12-16T05:52:49.0296607Z for a working request
- 57e4009a-381d-4153-8c7b-1b49ecec9175@2025-12-16T05:45:56.3561633Z for a slow request with no client timeout
- Region: Australia East

#### Repro steps

1. Deploy a Flex Consumption app with a single HTTP trigger
2. Send 10+ HTTP requests with short timeouts. In my case, clients have a 1.5s timeout. I've reproed with `curl`
3. Review logs - I've seen anywhere up to 10s if I remove the timeout

#### Expected behavior

Host always starts immediately and function is executed afterwards. Host usually takes <10ms and my function takes ~100ms.

#### Actual behavior

Intermittently, host initialization (`Initializing Warmup Extension.` log) starts 400ms-10s after a request is sent. This occurs despite 10 always-ready instances and very low RPS, eg 1 per minute.

In my case, clients are POSTing with a 1.5s timeout, causing 499 errors and lost data. Over 12 hours, I receive ~500 requests and ~50 of those fail.

#### Known workarounds

I really don't want to swap SKUs... Increasing the client timeout is not possible.

#### Related information

* Programming language used: Python 3.12
* Bindings used: Microsoft.Azure.Functions.ExtensionBundle.Experimental

Start by reproducing the Flex Consumption HTTP-trigger scenario with the listed curl steps and short client timeouts, then inspect the logs around “Initializing Warmup Extension.” Compare the working and slow invocation IDs and timestamps in Australia East. Done means identifying the cause of the delayed host initialization and preventing the intermittent 400ms–10s delay without requiring a SKU change.

Written by the indexing model from the issue text.