The standard format for RSA private keys is ASN.1 (https://www.rfc-editor.org/rfc/rfc8017#appendix-C) with the components encoded as INTEGERs. An INTEGER is always signed in ASN.1, so you need the leading 0 byte if the MSB of your positive number is set.

OpenSSL is just dumping the raw bytes comprising the value. Tools that don't show a leading zero in this case are doing a bit more interpretation (or just treating it as an unsigned value) to show you the number you expect.