This bug first discovered in Chinese Wikipedia, and it can be reproduced in MediaWiki Chinese-translated pages too.
To reproduce this bug, you can go to any pages that written/translated to Chinese, like this one:
https://www.mediawiki.org/wiki/How_to_report_a_bug/zh
then open the language variant dropdown to select another variant,
the available options become lesser and lesser.
It seems to be the options after the selected one are ignored
Although can change the URL manually to re-enter that "missing" variants, it is annoying.