The getClient method in GoogleAuth will ignore any credentials passed to it if it has cached credentials, regardless of differences between the credentials passed in the parameter and the credentials that have been cached:

https://github.com/google/google-auth-library-nodejs/blob/0ff24c7c4a1c5da604dd2019dfc225a922934e3e/src/auth/googleauth.ts#L656-L675

As far as I can find, there is no way to actually clear the cached credentials once it has been set. (Aside from manually explicitly setting the property to null, which doesn't feel right to me).

I'd imagine ideally that if the credentials didn't match, the cached ones would be ignored.
I'd expect at least some way to bust the cache; possibly as a property in the options parameter (something like useCachedCredentials: boolean).