there is indeed something strange in the update of Umbraco from v13.6 t ov13.7. that causes this.
I think its this change ( Cache null dictionary values by key by callumbwhyte 路 Pull Request #15576 路 umbraco/Umbraco-CMS 路 GitHub ) but someone who understands the caching of the repositories would have to understand why 
It appears to cause the lookup for the parent to fail when the parent has just been added previously (so it鈥檚 caching somewhere)
We can work around it (change here) - so we we will update things to do that, but also might put something against this change incase anyone can spot and fix it in the core,.