[ZFS] file in encrypted dataset disappears on first reboot

I found something that looks like a bug, however a comment was posted saying
"There must be some trivial explanation for this.".
Fair enough, only I have no idea what could be the explanation.

As I am a zfs novice I wonder if someone could offer some insight?
This only takes me a couple of minutes to reproduce, in case someone
wants to try to reproduce it.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261543