Portcheck has been broken for ipv6 capable systems since the DNS AAAA record was added back in 2017 https://github.com/transmission/transmission/issues/381. When the system resolves the AAAA record and uses ipv6 for the portcheck request, it receives "400 Bad Request" because the server side script at that URL is not ipv6 capable.
Migrated from https://github.com/transmission/transmission/issues/3102