PasswordGrant fails to decode user_name

Exception in thread "main" com.ocadotechnology.sttp.oauth2.common$OAuth2Exception: Client call resulted in error (200): Missing required field: DownField(user_name)
	at com.ocadotechnology.sttp.oauth2.PasswordGrantProvider$.$anonfun$apply$5(PasswordGrantProvider.scala:30)
	at cats.syntax.EitherOps$.leftMap$extension(either.scala:193)
	at com.ocadotechnology.sttp.oauth2.PasswordGrantProvider$.$anonfun$apply$4(PasswordGrantProvider.scala:30)
	at sttp.client3.monad.IdMonad$.map(IdMonad.scala:8)
	at sttp.monad.syntax$MonadErrorOps.map(MonadError.scala:60)
	at com.ocadotechnology.sttp.oauth2.PasswordGrantProvider$.$anonfun$apply$2(PasswordGrantProvider.scala:30)
	at sttp.monad.syntax$MonadErrorOps.flatMap(MonadError.scala:61)
	at com.ocadotechnology.sttp.oauth2.PasswordGrantProvider$.$anonfun$apply$1(PasswordGrantProvider.scala:31)
	at OauthTest$.main(OauthTest.scala:18)
	at OauthTest.main(OauthTest.scala)
Caused by: com.ocadotechnology.sttp.oauth2.common$Error$HttpClientError: Client call resulted in error (200): Missing required field: DownField(user_name)
	at com.ocadotechnology.sttp.oauth2.common$.$anonfun$responseWithCommonError$1(common.scala:103)
	at sttp.client3.MappedResponseAs.$anonfun$mapWithMetadata$1(ResponseAs.scala:91)
	at sttp.client3.internal.BodyFromResponseAs.$anonfun$doApply$2(BodyFromResponseAs.scala:24)
	at sttp.client3.monad.IdMonad$.map(IdMonad.scala:8)
	at sttp.monad.syntax$MonadErrorOps.map(MonadError.scala:60)
	at sttp.client3.internal.BodyFromResponseAs.doApply(BodyFromResponseAs.scala:24)
	at sttp.client3.internal.BodyFromResponseAs.$anonfun$apply$1(BodyFromResponseAs.scala:14)
	at sttp.monad.syntax$MonadErrorOps.map(MonadError.scala:60)
	at sttp.client3.internal.BodyFromResponseAs.apply(BodyFromResponseAs.scala:14)
	at sttp.client3.HttpURLConnectionBackend.readResponse(HttpURLConnectionBackend.scala:245)
	at sttp.client3.HttpURLConnectionBackend.$anonfun$send$1(HttpURLConnectionBackend.scala:56)
	at scala.util.Try$.apply(Try.scala:209)
	at sttp.monad.MonadError.handleError(MonadError.scala:24)
	at sttp.monad.MonadError.handleError$(MonadError.scala:23)
	at sttp.client3.monad.IdMonad$.handleError(IdMonad.scala:6)
	at sttp.client3.SttpClientException$.adjustExceptions(SttpClientException.scala:58)
	at sttp.client3.HttpURLConnectionBackend.adjustExceptions(HttpURLConnectionBackend.scala:295)
	at sttp.client3.HttpURLConnectionBackend.send(HttpURLConnectionBackend.scala:30)
	at sttp.client3.HttpURLConnectionBackend.send(HttpURLConnectionBackend.scala:22)
	at sttp.client3.FollowRedirectsBackend.sendWithCounter(FollowRedirectsBackend.scala:29)
	at sttp.client3.FollowRedirectsBackend.send(FollowRedirectsBackend.scala:24)
	at com.ocadotechnology.sttp.oauth2.PasswordGrant$.$anonfun$requestToken$1(PasswordGrant.scala:40)
	at sttp.monad.syntax$MonadErrorOps.map(MonadError.scala:60)
	at com.ocadotechnology.sttp.oauth2.PasswordGrant$.requestToken(PasswordGrant.scala:42)
	at com.ocadotechnology.sttp.oauth2.PasswordGrantProvider$.$anonfun$apply$3(PasswordGrantProvider.scala:29)
	... 6 more
Caused by: sttp.client3.DeserializationException: Missing required field: DownField(user_name)
	at sttp.client3.ResponseAs$.$anonfun$deserializeWithError$1(ResponseAs.scala:201)
	at sttp.client3.ResponseAs$.$anonfun$deserializeRightWithError$1(ResponseAs.scala:180)
	at sttp.client3.MappedResponseAs.$anonfun$mapWithMetadata$1(ResponseAs.scala:91)
	... 30 more

Originally posted by @reoxu in https://github.com/ocadotechnology/sttp-oauth2/issues/335#issuecomment-1376877020

Start with the failing stack-trace locations in PasswordGrantProvider.scala and PasswordGrant.scala, then inspect common.scala where the response is deserialized. Reproduce the reported PasswordGrant call using the OauthTest entry point and trace the response fields. Done means the call no longer raises the reported missing-field deserialization error.