Getting "unrecognized parameter 'promoa' " in Google Analytics

I'm getting the following warning when validating an event against the debug endpoint (https://www.google-analytics.com/debug/collect) despite promoa (https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters#promoa) being a valid parameter.

I've tried to set different values, like promoa="promo_click" and promoa="view" but it still gives this warning.

{
      "messageType": "WARN",
      "description": "Unrecognized paramteter 'promoa' found. Please see https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters#promoa for details.",
      "messageCode": "UNKNOWN_PARAMETER",
      "parameter": "promoa"
}

What can I do to fix this?

Solution

This seems to be a spurious message returned only from the debug endpoint. The live measurement endpoint seems to work ok, as the promotions appear correctly in the GA dashboard, under 'Conversions/Ecommerce/Internal Promotions'.

I reported this bug on the Measurement Protocol Issue Tracker in September 2020, and followed it up when they requested more information, but the last response that I got (via email) was on 7th September 2020, and seemingly nothing has been done to fix it since then.