Warning is not showing on SQL 2008, because it was not implemented in that release. It's that simple.
A big change in SQL 2014 was the introduction of the Cardinality Estimator. In many cases, this is for the better, but there are also situations where it can cause plan regressions, and this is what you may be seeing.
A first step is to enable trace flag 4199, to get access to bug fixes in the optimizer. It may be an issue in the CE that was fixed since the original release of SQL 2014. (And I take for granted that you are on SQL 2014 SP3 CU4.)