In views that don’t have data import enabled, Custom Dimensions values may be viewed for dates before the Custom Dimension was created.
You need to understand that Custom dimensions and custom metrics are like default dimensions and metrics in your Analytics account, except you create them yourself. You can use them to collect and analyze data that Analytics doesn’t automatically track. And Data Import lets you upload data from external sources and combine it with data you collect via Analytics. So, there is no way to see the custom dimension value before creating the custom dimension.