I have a case where a simple text field is turned into a drop down field when multiple entries are selected.
The Templates are the same, there are no values in the field for any of the documents yet.
Next, if I move the mouse over the field, it actually tries to populate the field and then removes the field input control entirely.
I tried moving the position of the field in the template around, but that didn't seem to have any effect. Other fields are just fine.
Below is a set of screen shots that show what's going on.
The last screenshot is of the field definition, as you can see, there's nothing special about it.