Thanks for your answer.
If I run your exemple with 2015 Q2 SP1 the reportsTo field is always the same regardless were I drop it.
I am currently using 2015 Q2 SP2 (v2015.2.827) wich I downloaded from UI for asp.net MVC 2015.2.805 but this version is not available in the dojo.
I just saw a bug fix regarding this in the beta version that I am unable to download http://www.telerik.com/support/whats-new/kendo-ui/release-history/kendo-ui-q3-2015-beta
TreeList drag&drop with autoSync enabled doesn't send corrent parent id field
I guess I'll just have to wait.