I see this problem at iPhone 12 at three browsers: Safari, Chrome and Yandex browser.
Moreover the previous version of my MediaWiki website (MW 1.40) - all looked good.

In T354076#9427211, @Aklapper wrote:

@Fokebox: Thanks for reporting this. For future reference, please use the bug report form (linked from the top of the task creation page) to create a bug report, and fill in the sections in the template, instead of deleting them.

Cannot reproduce, neither desktop nor my mobile phone:

Feel free to check https://www.mediawiki.org/wiki/Communication for support venues.

Link to site: https://www.wikijournal.org
Purpose/details about your project: Implementing maps to articles about tourism and others.
Wikimedia Affiliate supporting project: No support from Wikimedia Affiliate project

In T261921#8062358, @matmarex wrote:

Try the workaround which is now documented at https://www.mediawiki.org/wiki/Extension:VisualEditor#Allowing_VisualEditor_on_page_titles_containing_slashes.

If you can't do that, wait for MediaWiki 1.39 later this year, in which VisualEditor will no longer use the loopback network requests causing all these issues (T305108).

Are there a solution of the problem as I have faced with issue editing, saving pages with slash in its title?

@Jdlrobson yes I have $wgVectorResponsive = true;

@Aklapper my project is not supported by Wikimedia Affiliate. Just wondered if I can use Wikimediamaps with Karographer extension/

I have the same problem at my wiki:
Продукт Версия
MediaWiki 1.35.1
PHP 7.4.20 (cgi-fcgi)
MySQL 5.7.34
ICU 57.1