Chris Webb shows how to use a parameter on the Table.AddColumn function in M to define specific data types:
In the last two screenshots the ABC123 icon in the column headers show that they are set to use the Any data type; the columns returned by calling the function have lost their data types.
The key to solving this problem is using the optional fourth parameter of the Table.AddColumn() function, which allows you to set a data type for the column that function adds to a table. Altering the Invoked Custom Function step of the previous query to do this, setting the new column to be a table type like so:
Worth reading in its entirety.