Getting error Missing Parameter: redirect_uri even when a redirect URI is present

Getting this error:
{"error":"parameter_absent","error_description":"Missing Parameter: redirect_uri"}

This error occurs when using Micropub (tried with ownyourgram and Teacup) and when trying to log in to the Indieweb wiki. But there is in fact a redirect URI - for example, when using Teacup, this is in my address bar:

http://kimberlyhirsh.com/wp-json/indieauth/1.0/auth?me=http%3A%2F%2Fkimberlyhirsh.com%2F&redirect_uri=https%3A%2F%2Fteacup.p3k.io%2Fauth%2Fcallback&client_id=https%3A%2F%2Fteacup.p3k.io&state=97bd85bbcb5deb96&scope=create&response_type=code

Start by reproducing the reported Micropub or IndieWeb Wiki login request using the example Teacup URL, then trace where the WordPress plugin reads and validates redirect_uri. Confirm whether the parameter is accepted from this request format and identify the failing validation path; done means the request no longer reports redirect_uri as missing.

Written by the indexing model from the issue text.