This is happening from https://gerrit.wikimedia.org/r/#/c/426842/. One minor thing that I noticed, The value of text field is missing where Original one has. There is need to add 'default' => str_replace( '_', ' ', $from ), in prefix array.

Change 431281 had a related patch set uploaded (by VolkerE; owner: VolkerE):
[mediawiki/core@master] [WIP] Special:PrefixIndex: Override subpage in form field

https://gerrit.wikimedia.org/r/431281

Quick Hack, this originally comes from https://gerrit.wikimedia.org/r/#/c/305315/, I have tested it. It works for me as expected. This needs some minor improvements :)

   function execute( $par ) {
		// Fetch the prefix term
		$prefix = str_replace( "\n", " ", $request->getText( 'prefix' ) );

		if ( strlen( $par ) && !strlen( $prefix ) ) {
			$query = $request->getValues();
			unset( $query['title'] );
			// Strip underscores from title parameter; most of the time we'll want
			// text form here. But don't strip underscores from actual text params!
			$query['prefix'] = str_replace( '_', ' ', $par );
			$this->getOutput()->redirect( $this->getPageTitle()->getFullURL( $query ), 301 );
			return;
		}
   }

@Jayprakash12345 Would you amend my patch with your proposed solution or add a new patch? I'd remove the task reference in the regression patch.

In T193927#4185317, @Volker_E wrote:

@Jayprakash12345 Would you amend my patch with your proposed solution or add a new patch? I'd remove the task reference in the regression patch.

Ok, will do

@Jayprakash12345 can I take up this issue, it will be my first contribution.

@amankh_99 You're welcome. Just Claim the task by Add Action... → Assign / Claim and then Submit.

Also, See https://www.mediawiki.org/wiki/Category:New_contributors.

@amankh_99: Please see https://www.mediawiki.org/wiki/How_to_become_a_MediaWiki_hacker#Feedback.2C_questions_and_support - thanks!

Change 431281 had a related patch set uploaded (by Bartosz Dziewoński; owner: VolkerE):
[mediawiki/core@master] Special:PrefixIndex: Fix regression on prefix input value

https://gerrit.wikimedia.org/r/431281

Is it possible that the regression mentioned here is the cause of T194655?

Change 431281 merged by jenkins-bot:
[mediawiki/core@master] Special:PrefixIndex: Fix regression on prefix input value

https://gerrit.wikimedia.org/r/431281