Because you have a keyDown event listener on all inputs that match the selector
input:not(.allowInput), textarea, select
blocking non-numeric input.
This of course matches the fullName field.
The only reason it appears to work after a refresh (which isn't always true, only sometimes) is because you have a race condition, so it's possible for the Marketo form to be finished loading after the keyDown event binding, in which case the newly injected form fields won't be restricted.
Thanks Sanford! I will apply the change today and try it out.