OAuth Token Refresh 401 Failure in AppFoundry Premium App

Struggling to figure out why our Genesys Cloud AppFoundry Premium App receives a 401 Unauthorized error during silent token refresh for multi-tenant orgs. The initial authorization works, but subsequent refreshes using the refresh_token grant type fail with invalid_grant. We are using Node.js SDK v2.1.0. Here is our current OAuth configuration:

auth:
 grant_type: refresh_token
 scope: readonly agent:manage

Any insights?