Sporadic "SenderId mismatch" errors?

[Doug Nebeker]

I'm getting SenderId mismatch errors on maybe 10% of the push messages that go out via FCM to an Android app.  From everything I've read this happens when the project's credentials .json file doesn't match.  But in that case shouldn't I expect 100% failure?

Could there be a mismatch between the user's registration token and the sender ID?  How would I ever fix that?

Thanks for any ideas.

Doug