Hi you should probably ask your question at the contact form7 forum here: https://wordpress.org/support/plugin/contact-form-7/
For the record, I don't see the white line you show in your image when using the chrome browser.
I don't see it either on desktop or mobile. Clear device cache and site cache and see if it's still there and if not as per suggestion above, try contact form 7 support. Also check the from settings itself in case you missed something there.
