I was surprised to read this, so I tested it.
I made a Dutch parent with a Dutch text as child, then made an English translation without a parent. After that, the Dutch text no longer had a parent. The parent still existed. I could make the Dutch text the child again, however.
Hmmm... I'm not sure why you did this. What if the English site has a totally different structure than the Dutch site? With every translation we would have to control the original structure again... I think Polylang should only set the relationship between translated items.
I'm relatively new to WP though. Maybe I don't see the whole picture, but to me this sounds like a strange choice.