Previously, due to formatting, the field was truncated at 255 characters.
Now all text is being imported but is too large to fit in the space available.
On your form, you can increase the width of the listbox column holding that field but because its a listbox you can't show the text on multiple lines. However you do see the full text in the textbox below the list.
You shouldn't be viewing data directly in the table but if you really want to do this you can increase the row height to show all the text.