The platform-admin ui uses the appAuthHelper library to managed tokens. When a token has expired it attempts to renew it. The way it knows to renew the token is by looking for the "invalid_token" error in the "www-authenticate" header. The upload endpoint is missing this header.
The header should look like this =>
Bearer realm="IDM",error_description="The access token provided is expired, revoked, malformed, or invalid for other reasons.",error="invalid_token"