I suspect this is due to a feature being enabled for canonicalization and that the key part is the presence of the colon rather than the port number. On a 10.15.4 system, I see a line in the debug output which is not present in the screenshot:

> debug1: resolve_canonicalize: hostname example.org:7999 is an unrecognised address

If instead I use `-p` or a config-file option, everything works as expected.