That is the correct expiry time of the intermediate certificate (the Let’s Encrypt Authority X3 certificate in the chain you’re viewing there).
Since it’s being displayed here as the expiry time of the leaf certificate for zerossl.com, I think you’ve found a Safari bug.
Do you have a way that you could export a copy of the certificate in PEM format (perhaps from within the Details tab there or something)? Maybe we can interest someone at Apple in looking into this.
Edit: also, what version of Safari and on what version of macOS?