Out of curiosity, do the items selected have a different value in the field you are looking at? This will cause the field to show blank in MPROP. 

The first screenshot below shows multiple items selected with the same value in the Line Number custom data field. The second, shows changing the value on one of the parts from 5206011 to 5206012, and the third shows the Line Number custom data field as blank in MPROP. The value is not blank on each of the parts, it is simply not the same between the parts that are selected.