Please note, that the mentioned by you version of the controls (2015.1.310.40) is an internal built which is not fully tested. Can upgrade to the the official release published after this build (2015.1.401.40) and see whether the problem persist?
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.