Hello Elegant. I realize this is about 2 1/2 years later, but I am having the same issue, and I am wondering if/how you resolved the issue.
In my case, I have three child data sets, but his could be a useful clue: Two that were set up initially on TrueNAS Core V11 or 12 (I don't remember), and a third set up recently on V13. In my case, "child_a" and "child_b" seem to work as expected, but "child_c" has the stale handle issue, with the story being very much similar to what you describe in this post.
I am about to give up on the child dataset approach for this and use folders and ACL permissions instead, but I would very much like to understand.
Like you, I am now wondering if this is a bug instead of configuration issue.
Code:
vol1/parent/child_a (works, set up a couple years ago on V11 or 12)
vol1/parent/child_b (works, set up a couple years ago on V11 or 12)
vol1/parent/child_c (mounts, but get a stale file error. Set up recently on V13)