Go http server is sending Alert message: decryption_failed_RESERVED in TLS 1.2 but as per RFC 5246 it is not correct behavior

We have http server in golang and it is sending Alert message(decryption_failed_RESERVED(21), with TLS 1.2 but this is not compliant with TLS 1.2 RFC 5246, why this is happening and how to come out of it.