I did this

server:

echo -e "HTTP/1.1 200 OK\nStrict-Transport-Security: max-age=31536000\nContent-length: 0\n" | socat STDIN OPENSSL-LISTEN:8443,certificate=cert.pem,reuseaddr,verify=0

client:

curl --insecure --hsts foo.hsts https://<ipaddress>:8443

Note: --insecure is used here to avoid need to set up PKI for the PoC, it doesn't change the outcome however.

afterwards foo.hsts:

# Your HSTS cache. https://curl.se/docs/hsts.html
# This file was generated by libcurl! Edit at your own risk.
<ipaddress> "20220528 21:22:30"

Also later the HSTS applies to the IP address, while it should not.

I expected the following

libcurl implementing https://datatracker.ietf.org/doc/html/rfc6797 design, specifically:

   4.  HSTS Hosts are identified only via domain names -- explicit IP
       address identification of all forms is excluded.  This is for
       simplification and also is in recognition of various issues with
       using direct IP address identification in concert with PKI-based
       security.

curl/libcurl version

curl 7.77.1-DEV (x86_64-pc-linux-gnu) libcurl/7.77.1-DEV OpenSSL/1.1.1k zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) nghttp2/1.43.0 librtmp/2.3 OpenLDAP/2.4.57
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli HSTS HTTP2 HTTPS-proxy IDN IPv6 Largefile libz NTLM NTLM_WB PSL SSL TLS-SRP UnixSockets

operating system

Linux anonymized 5.10.0-6-amd64 #1 SMP Debian 5.10.28-1 (2021-04-09) x86_64 GNU/Linux