FreeBSD i386 cannot be configured to run TLS on Sendmail. FreeBSD arm64 works fine using the identical instructions. OpenSSL will generate a self signed certificate if it is defined in /etc/rc.conf for sendmail but sendmail errors out when attempting to load it.
Sendmail got some TLS-related fixes after 12.1-RELEASE. Could you please verify if the problem is still relevant for 12-STABLE?