Datetime value is reset to its default value after editing another xxx2many field
I found this issue in !69 but it can also be reproduced using `attendance` module.
Steps to reproduce:
1. Create an attendance record
2. Update the value of the `at` field by changing any value
3. Clear the `employee` of the record, open the search window and set another employee
Current behaviour:
* The value of the `at` field is updated back to the default value
Expected behaviour:
* The `at` field is keep at the modified value after updating the employee field.
I was able to reproduce the issue on `6.0`, `6.4` and current development series
issue