OpenSSL and Secp256k1 differing in implementation of ECDSA signatures?
The code you are referring to in libsecp256k1 is not for ECDSA.

It implements the custom compact signatures that Bitcoin Core uses for message signing and verification.

The normal ECDSA code in libsecp256k1 should be identical in acceptance to the one in OpenSSL (apart from the fact that by default, it only accepts and produces low-s signatures, as a way to reduce malleability potential).