I have made some tests by using live connection SSAS tabular as data source. However I cannot access date columns in DAX under my dimension. I can only access the measures.
As you can see in this article: Using Analysis Services Tabular data in Power BI Desktop. When connecting live, no data from the Tabular model is imported into Power BI Desktop. Each time you interact with a visualization, Power BI Desktop queries the Tabular model and calculates the results you see.
So How did you access the date columns in DAX? What kind of measure did you create? Could you please share us more information like some screenshots of your report or the measure which contains the date columns if possible?