I've just noticed that chrony isn't linked with the nettle library despite nettle being enabled as default.
If you look at the build log from poudriere ( http://beefy6.nyi.freebsd.org/data/latest-per-pkg/chrony/3.5/120amd64-default.log ) it shows the port installing nettle but then during the configure stage it says nettle is not found and SECHASH is disabled.
===> chrony-3.5 depends on shared library: libnettle.so - found (/usr/local/lib/libnettle.so)
Checking for nettle : No
Features : +CMDMON +NTP +REFCLOCK -RTC +PRIVDROP -SCFILTER -SIGND +ASYNCDNS +READLINE -SECHASH +IPV6 -DEBUG