I can't seem to replicate this and don't know if I'm looking in the right place -- do you have screenshots?
Also, I'm not clear I understand why autocapitalization is undesired here

You'll need a device with a virtual keyword to replicate https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/autocapitalize

On some projects case sensitivity matters and lowercase article titles are more common so this can be in particular a big hassle for editors.

Here's a screencast of my Android capitalizing words unexpectedly:

This seems to be a UI change on mobile, which should probably be done by the Mobile team. The change in input make sense to us (the Search Platform team). If you need more work from our side, please reach out!

There is no mobile team. The fix here needs to be done by whoever supports the Special:Search page. I thought this was the search platform team?

I think the specific change required here is to pass autocapitalize parameter to the SearchInputWidget widget here: https://github.com/wikimedia/mediawiki/blob/446d11dfba317108ac79b54107288ef580af26d7/includes/search/searchwidgets/SearchFormWidget.php#LL121C23-L121C40

Change 917392 had a related patch set uploaded (by Ebernhardson; author: Ebernhardson):

[mediawiki/core@master] Set autocapitalize on Special:Search input

https://gerrit.wikimedia.org/r/917392

Change 917392 merged by jenkins-bot:

[mediawiki/core@master] Set autocapitalize on Special:Search input

https://gerrit.wikimedia.org/r/917392

This looks to now work as expected on en.wiktionary.org, I'm going to boldly assume it works everywhere else as well.