If you show a prototype machine to a manager or customer, they can see the duct tape that holds it together. No sensible person would rely on such an obviously unready machine for production work.
While it’s hard enough to see the digital duct tape that holds prototype software together, we often hide that same haphazard work behind a polished UI. It should come as no surprise when managers ask us to rush such apparently finished code into production.
Show the duct tape. Avoid UI polish. Get the designers on board with sketching first and only refining after the prototype works. Document those un-addressed edge cases (and those frequent but skipped cases). A manager who makes the conscious decision to request more polish will take ownership of that decision and leave you to do right by the work.
(Based on my original post at vulpine.club.)