what gets displayed is the same template on the page.
difference being that on COD purchases the content gets dynamcially “replaced” (as there is no need for a page reload) whereas when returning from epay, the page gets loaded entirely (obviously)
seems to me that your css might be targeting a container that exists on the order form page but does not exist when coming to the page back from epay
essentially, you should be targeting .wppizza-cart-thankyou .some-element{/*some css*/}