This seems strange to me - any version of ZFS recent enough to support receiving encrypted snapshots should support mounting them (which, AFAIK, means "OpenZFS-running FreeBSD implementations" - I don't believe the non-OpenZFS, integrated implementation ever got encryption support of any kind).

I've seen several people report using OpenZFS encryption on FBSD on various mailing lists, so I'm 95% sure it's not secretly broken on there.