It appears missing.php has no handling for URLs ending with /wiki/foo well. See https://cs.wikivoyage.org/w/index.php?title=Belgie vs https://cs.wikivoyage.org/wiki/Belgie for an example.

I expect that either both of them redirect to https://incubator.wikimedia.org/w/index.php?title=Wy/cs/Wikivoyage:Hlavní_strana (ie. main page of the Czech Wikivoyage) or both of them redirect to https://incubator.wikimedia.org/wiki/Wy/cs/Belgie (the page about Belgium).

The redirection happens at https://github.com/wikimedia/operations-mediawiki-config/blob/master/wmf-config/missing.php#L49, and should likely handle /wiki/XXX as well.