The new issue forms looks really great. Just a little addition to it:
at the bottom, you could add a short summary: 1 / 3 required fields are filled out

This summary could change, depending on the status:
if all required fields are filled out, it's green with a check.

If there are missing required fields, it could be red or orange and you can click it. Then it will jump to the first / next missing required field.

This way it could improve the UX even more, because people have a status and see directly why the can't click on the "Submit new issue" button.