Right. I'm more suggesting that this combination of options should be handled differently inside of libcurl. E.g. either you want the native store for validation, or you don't. If you "also set a CA certificate file" as per the libcurl docs, presumably you want to use that cert only, and no others.
I'm not too familiar with how the library works internally, but I was imagining something like the above would be possible.
It isn't (lib)curl that's doing this, it's Apple's own version of LibreSSL that curl is built against on macOS.