Steps to reproduce, actual outcome, and expected outcome welcome. See https://www.mediawiki.org/wiki/How_to_report_a_bug
What exactly is expected when entering [https://www.mediawiki.org/ test]?
That at any point in plain text, for example after the character [, an URL could start in the topic title, so the parser should find URLs and turn them into clickable links?
That Flow should support and recognize classic wikitext markup ([protocol://externallink linktext]) in the topic title and interpret it?
[[test]] is an internal link, [[en:test]] too. Link to another wikis are not considered as external, except if they are written as real external links, like [https://www.mediawiki.org/ test].
Wikisyntax, except for internal links, is prohibited from titles to avoid mess in notifications.
Please also reply to the follwing questions @Bugreporter, thanks!
See also T15205: Ability to use hyperlinks in log summaries and T16892: Linkify plain external links in revision editsummary and log reason. We can't reasonably support external links in Flow topic titles unless they are also supported in edit summaries. It sounds like there were some spam concerns with those requests.