There is indeed something very odd occurring.
1. Using the Edge browser the line did not appear at first.
2. Pressing the Ctrl key while scrolling forward with the mouse to enlarge the contents of the Edge window reveals the white line under the submit button.
3. At various levels of such enlargement a dark line appears below the form, and the white line sometimes appears but occasionally disappears.
4. It seems not entirely connected with the amount of such enlargement because, using Ctrl+0 to restore the browser contents to the default size seems to leave the white line there. Therefore the appearance or non appearance of the while line seems unconnected with it's pixel height but with the OCCURRENCE of change in viewing size.*
5. When the white line appears it seems always to be exactly the same width as the fields in the form. I attach a full-screen grab at one size, suggesting that it is something like a ghost field, perhaps replicating part of one of the fields above.
* Doing the same in Chrome did not replicate this, but the enlargement of the contents in the Chrome window did produce the line.
My feeling is that it would be worth seeing what happens if just the form were replicated in a separate project file. If the problem proves not to occur in that separate project, the problem would seem to relate to other aspects of the page we see. If it does occur in the separate project file, that file could be referred to the CoffeeCup team to see whether they can cast some light on the matter.
Frank