Model keeping snapshot stopped working after retrieve related model

It works

$comment = Comments::findFirst(1);
$changed = $comment->hasChanged('text');

It doesn't work

$comment = Comments::findFirst(1);
$post = $comment->post;
$changed = $comment->hasChanged('text');

Error: Change checking cannot be performed because the object has not been persisted or is deleted

Both model set to $this->keepSnapshots(true); in initialize();