If using Modify Data Master function and not changing one or more attributes, say something other than "undefined" in the final expression.
I tested the Modify function and it does allow you to update the value of a single attribute in a data master without affecting the values of the other attributes. That's great! However, once you hit OK, the expression shows 'attribute =' 'undefined' anywhere you didn't include something. This misled me into thinking that it was going to wipe the data for each of those attributes.
One potential solution would be to simply remove any mention of those attributes in the final expression. That would have given me more confidence and understanding of how the function works.