I cannot explain this. Something for the MVP's with knowledge of the actual SQL code, but I do have a solution.

I believe this is a legacy issue with the check constraints from the days of rules and binding (sql7 if memory serves)....The Check constraint is only being evaluated when then Id (the column in the check constraint definition) is being inserted or updated.

The work around is to add the ISEnabled to the Function

CREATE FUNCTION udfTemp( @Id int, @IsEnabled Bit)

This way when you try the update it will fire the Check and fail