I believe I've found a bug in gridview. After adding a childtemplate (1) to a childtemplate (2) of the mastertemplate, any change to the grid will 'disconnect' childtemplate (1) from childtemplate (2).
To work around, add the following in your *.designer.cs after the changes:
Of course, this is incredibly annoying to do every time you change the grid, so I'm hoping another solution is possible.
Version: v. 2016.1.216.40