Filled in a fourteen-field form, hit submit, got an error on field…

Filled in a fourteen-field form, hit submit, got an error on field two, and the other thirteen were empty.

Preserving form state across a failed submit has been solved for as long as there have been forms. It is not hard and it costs nothing. It is simply never checked. The person who built it tested it by succeeding.

That is the root of most of this. The happy path gets walked a hundred times. The failure path gets walked once, by someone who already knows the right answer and types it correctly.