We found the problem, it's related to "complex types" and nullables.
Our DTOs has some non-nullable enums that, with EditTemplate, breaks everything.
In fact, using the same example that you give us and modifying the class into this (notice the Test property)
raise the bug. If you put nullable with "?" notation everything works.
The annoying thing is that if you leave it non-nullable and remove the EditTemplate the cell itself goes to edit mode and saves everything without any problem!
Maybe it will be better to improve serialization data for grid.