curl overwrites HSTS cache entries if requests are performed in parallel.
curl --parallel --hsts hsts.txt https://site1.tld https://site2.tld https://site3.tld
Only one of the sites contacted will have entry in hsts.txt
afterwards. Non-TLS connection to the other sites will not protected by TLS.
Request performed over insecure channels unexpectedly and loss of confidentiality and integrity.