I previously reported this issue 2 days ago, thinking it was a network error in downloading pkgs. Since that time, I have tested nextcloud and plex official packages, and community package gitlab, all fail in the same way.
I've attempted to eliminated network download issues with pkgs by confirming that the pkgs are present and stored in the jail's cache (/mnt/jails/iocage/jails/plugin-jail/root/var/cache/pkg) during the installation, and after failed installations.
All plugins fail after multiple attempts to install their packages, and never reach the point where they would run the post install script.
messages in a jail' log/messages look like: