From within my iOS app these open the correct page (for the film "&ME") via openURL:
wikipedia://en.wikipedia.org/wiki/&ME
https://en.wikipedia.org/wiki/&ME
https://en.wikipedia.org/w/index.php?search=%26ME
But these don't (they open some other, i.e. error, page):
wikipedia://en.wikipedia.org/w/index.php?search=&ME
wikipedia://en.wikipedia.org/w/index.php?search=%26ME
wikipedia://en.wikipedia.org/w/index.php?search=%2526ME
wikipedia://en.wikipedia.org/w/index.php?search=%25%26ME
Searches not containing '&' work fine.
If you've solved this, what did you do? Or can you suggest another way to escape ampersand I haven't tried? Or does this look like a bug in the iOS Wikipedia app?