The correct result is 'ß', incorrect results are 'SS' or 'ẞ'. Chromium 54 gives me 'SS', Firefox and Edge are OK.
|mediawiki/core : master||Add exceptions in mw.Title where mb_strtoupper doesn't match String.toUpperCase|
Comparing the length only works if the length changes. The article on 'ß' says it is unique in not having a capitalisation, we should really write a script to compare the output of mb_strtoupper and toUpperCase and come up with a mapping for any other cases that fail, but this may the only sensible case that fails.