Native android app WebAuthProvider.init successful login but Credentials object all null values

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I have an application that has been running for a couple of years, and the authentication code was written a few years before that,
so I know my auth0 library and code calls are really, really old, but … if it ain’t broke - don’t fix it

unfortunately Android is forcing me to upgrade from taget SDK minimum 34 to 35 (and google billing library 7)
ok so I made those changes (none of which at all affects the auth0 authentication i think)
and it wasn’t broke but now it is

under certain circumstances my successful logins are returning a credentials object on Success but every field is null instead of the
values from the login success

USING

implementation ‘com.auth0.android:auth0:1.8.0’

CALLING

WebAuthProvider.init(auth0)
.withScheme(scheme)
.withAudience(audience)
.withScope(“openid offline_access email”)
.withParameters(parameters) // “prompt”, “login”
.start((Activity)context, new AuthCallback() {

    ...

@Override
public void onSuccess(@NonNull Credentials credentials) {

funny thing is I have different build types in my build.gradle

the one for general running and debugging still works fine
the one to create a release to go to the Play Store is the broken one

the config for these build types is IDENTICAL with regard to the auth0 client, domain, audience, scheme, everything
yet one works as it did before (credentials comes loaded) and one comes back empty

the ONLY difference in the build types are these three lines for the type that is failing

     minifyEnabled true
     shrinkResources true
     proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

I am struggling to see how any of the changes I made or the three lines above could cause a difference in AUTH0 behavior like i am seeing ?

any ideas where to look next would be most welcome