Another customer had this same issue. I was mid way through investigating it, but was waiting to hear back about something else. I’ve just checked the issue again. The other customer was using WP Forms Lite. And I can see that the CSS file loads in the footer:
But for some reason the browser isn’t successfully loading it. This could be due to some invalid code anywhere on the page. I’m not sure at this stage. But if there is an option to load the CSS for the form in the head, that might fix the issue for you. Even if you just enable this temporarily to make style changes in MT with the form looking right.