Function proxy receives 429 response from downstream function app

Our function proxy app occasionally receives a 429 response code from a downstream function app.
The downstream function app is not hitting any concurrent HTTP limits (it only gets 3 total requests per minute). The request can be seen as a failed dependency of the function proxy app in Application Insights logs. The HTTP request is not visible in the logs of the downstream function app. Most of the requests to this app succeed, but we occasionally see a batch of 429 responses.

#### Investigative information

Please provide the following:

- Timestamp: 12/15/2020, 11:57:25 AM (UTC)
- Function App version: 3
- Invocation ID: 313a6f27-e40c-41e4-aa54-d9f285fde375
- Region: Canada Central (also noticed happening in UK South in other function proxies/apps)

#### Repro steps

Make an HTTP request to the Function Proxy app.

#### Expected behavior

The proxy app forwards the request to a function app.
The downstream function app handles the HTTP request and returns a 200 response code.

#### Actual behavior

The downstream function app returns a 429 response code.

#### Known workarounds

None.

#### Related information

The downstream function app is a compiled C# function app. The same code is deployed to other regions and is working OK there. The 429 response code is only received occasionally, with the majority of requests being handled successfully. There are very few requests to this app and there is no way we are hitting concurrency limits. Memory and CPU look to be stable throughout.