Oauth authz code flow does not redirect user back after sign-in

Our service is using the oauth authorization code flow and started to seeing issues since last week.
the normal flow looks like:
Service direct user to twitch “oauth2/authorize” endpoint with the required params
→ user type in the user name password
→ user types in the a verification code (from SMS or email)
→ redirect back to return url (our service)

in our case, the last step which redirects back to redirectUri is not happening, the last related call we found is:
“POST https://passport.twitch.tv/login HTTP/1.1”, which returns 200 with the content:
{“access_token”:"---------------------------",“redirect_path”:“https://www.twitch.tv/”}

this is the last related redirect_path I found in the fiddler trace and it is not our own redirect URL configured or passed in to the API.

there is no recent change to our service or app setting, just want to see if there is anything known issue or guidance on how to debug

I have the trace but it contains some sensitive info in there, I can provide any required info needed

thanks
jy