Make sure you've formatted the cells as text in the table you are using to make the form.
Since you can switch modes/keyboards when entering data to the table, this is easily overlooked; but it determines what keyboard will pop up when you select teh field in a form.
You can change an entire row or column of cells at once. Just select the table, then tap the gray bar opposite of the column or row to select it all. Go to the controls (the i icon), select the format tab, and choose Text.
Headers are formatted as text by default. All other cells are formatted numeric by default (though that does not show in the control panel).