I looked into this. The variants were never shown by ULS itself. On mobile, the old language button opened MobileFrontend's own language overlay, not ULS. That overlay read variants directly from the page DOM (the #p-variants element that MediaWiki renders for wikis with language converters like Chinese), and added them to the "Suggested languages" list. The code is in MobileFrontend/src/mobile.startup/PageHTMLParser.js (the getLanguages() function).

When the ULS rewrite beta is on, MinervaNeue gives the click to ULS V2 instead of opening MobileFrontend's overlay (see initMobile.js in MinervaNeue, the wgULSLanguageSelectorV2Enabled branch). ULS V2 then builds its list by reading only #p-lang / .interlanguage-link-target. It does not read #p-variants. The rewrite has no code for language variants at all.

It is a missing feature in the rewrite. To fix it, the rewrite needs to:

1. Also read entries from #p-variants li a when building its language list (in getLanguageNodes() in resources/js/ext.uls.interface.js).
2. Show them in a sensible place. "Suggested languages" matches the old behavior. Variants are not real interlanguage links, so they may need a small adapter as they do not have the same hreflang and autonym fields.

Note that making this change would affect both the desktop and mobile versions of the language selector.