As stated before, you can’t simply place a GUI web app under a path, see explanation.
Your first request /portainer will be forwarded as request to /, Portainer will respond with a page. Browser will then request /vendor.js as part of the initial page, Traefik doesn’t know where to route it, because is has no /portainer prefix, will respond with 404.
But it seems Portainer does support setting a base path (doc), so just use that, don’t strip it. Traefik dashboard does not support a base path, use a sub-domain.