mapping rule defined on backend is not matched if url contains multiple slashes

I've defined product-backend object with path '/pokus' and backend with mapping rule '//hokus' which should create url like 'https://hostname:port/pokus//hokus' which is valid but doesn't work. In attachment you can find Apicast debug log named 'f' and proxy configuration named 'g'.
Multiple slashes included in url without any delimiter are OK, see https://tools.ietf.org/html/rfc3986#section-3.3