For testing I created a custom list that contains one User field, added the event receiver and overrode all the list item event handlers. The test is repeated when using Classic authentication and when using Claims based authentication.For every event I tried to get the value of the user field from properties. Also the test is repeated for Share Point 2010 and Share Point 2013.
From the results it turns out that when getting the value of a user field from After Properties in Item Adding and Item Updating events the value is different when using the New and Edit form from the value when adding and updating the field pragmatically.
This difference is present only when using Claims based authentication.
If so, I think the problem may caused by something incorrect in the Form View's aspx template.
Would you try comparing the problem page with the other page which works correctly to see whether there is any diference?
When accessing a user field in event receivers there are few differences in the returned values when Classic mode authentication is used from the value when Claims based authentication is used.