BigQueryReadClient::ReadRows fails with `UNKNOWN: Error occurred when fetching oauth2 token` after CreateReadSession succeeds

Client `google-cloud-cpp v2.22.0`

## What Happened
```
Error Source: RUNTIME
Error Code: INVALID_STATE
Reason: BigQuery ReadRows failed for stream projects/rising-woods-374602/locations/us/sessions/CAISDG80QTJWdmc5UnpMSRoCamMaAm55/streams/CM8RGgJqYxoCbnkoAg: UNKNOWN: Error occurred when fetching oauth2 token. (rows delivered by this data source before the failure: 0)
```

I'm not filing this as a bug yet. I want to understand the failure mode.
1. Under what conditions does this occur?
2. Is this a client side or service side issue ? how do I confirm which one it is?

Is `UNKNOWN` error type retried by the default `BigQueryReadConnection` retry policy? If a transient token fetch failure lands as `UNKNOWN`, it looks like it would bypass retry and kill the read outright. Is that expected, and is there a supported way to make token-fetch failures retryable ?