Previous members can no longer log in to an ArcGIS Online organization via OpenID Connect (OIDC) and receive the error message, "The username you entered is not a member of this organization. Please request an invitation from its administrator."
The issue was due to a user error; the customer's identity provider (IDP) management team reconfigured their external OpenID Connect (OIDC) provider to send a different value for the sub-claim, resulting in new accounts being created.