I spent about 10 minutes playing with pkg_add -D SIGNER= and could not get it to fail, so I extracted the signify message from the gzip comment header in the 6.6/i386 package for upobsd-1.1.tgz, confirming what you see in the package SHA256.sig file stored with the collection.
Code:
untrusted comment: verify with openbsd-65-pkg.pub
RWS5D4+188RI6tcXF+2EEvhE2KaknlwzHYIPEgqnls9+3BACLLxe4++D34iyxhYStsva7nrwylSh0yGGnAsJKypY8gJLsft0ZgQ=
date=2019-10-11T14:55:13Z
key=/etc/signify/openbsd-65-pkg.sec
algorithm=SHA512/256
blocksize=65536
You may wish to bring this to the attention of the Project via the ports@ mailing list.