Why would SQL Server stop excluding partitions appropriately?
In the second execution plan, the seek predicate has a CONVERT_IMPLICIT function in it. This means the wrong data type was input and SQL Server has to convert the data type to something else. In this case, it was expecting a DATE data type and something other than DATE was passed in... most likely a (VAR)CHAR or N(VAR)CHAR value. The fix would be to either explicitly cast/convert the value before submitting the query or fix the code to use the proper data type.

Here are some further resources on the matter:


Implicit_Conversions 
Conversion Matrix
Performance Issues with Convert_Implicit