Power BI Measure Breaks Parent/Child Relationship

Question

I have a report with a parent/child relationship between 2 tables. When I select the parent in the first table, the associated child objects are successfully displayed in the second table.

Now I've created a measure to calculate a value in the child table. When I add that to the child table it returns all child rows, regardless of what is selected in the parent table. The code is as follows:

Measure 2 = 7-MONTH(SELECTEDVALUE('Child'[Estimated Date]))

Interestingly, the following code works just fine and returns the appropriate child rows:

Measure 2 = MONTH(SELECTEDVALUE('Child'[Estimated Date]))