My initial assumption about faulty documents is wrong (thanks @ EBernhardson)

The query has filter=translated parameter, which causes CrossLanguageTranslationSearchQuery to be invoked. This returns dynamically constructed documents that don't have all fields. Generally this is not a problem, because these messages are treated as local in this code path:

			$handle = new MessageHandle( $title );

			if ( $handle->isValid() ) {
				$uri = Utilities::getEditorUrl( $handle, 'search' );
				$link = Html::element(
					'a',
					[ 'href' => $uri ],
					$this->msg( 'tux-sst-edit' )->text()
				);
			} else {
				$url = $this->urlUtils->parse( $document['uri'] );
				if ( !$url ) {
					continue;
				}
				$domain = $url['host'];
				$link = Html::element(
					'a',
					[ 'href' => $document['uri'] ],
					$this->msg( 'tux-sst-view-foreign', $domain )->text()
				);
			}

In the case of error, $handle->isValid() returns false.

Best course of action seems to make the code more robust and add more logging.

Change #1098502 had a related patch set uploaded (by Nikerabbit; author: Nikerabbit):

[mediawiki/extensions/Translate@master] SearchTranslationsSpecialPage: Add more logging for T380961

https://gerrit.wikimedia.org/r/1098502

Change #1098502 merged by jenkins-bot:

[mediawiki/extensions/Translate@master] SearchTranslationsSpecialPage: Add more logging for T380961

https://gerrit.wikimedia.org/r/1098502

After deployment we should get more info to investigate further.

The translation unit in question is Translations:Movement Charter/Drafting Committee/Newsletter/1/1/ja and it belonged to translatable page https://meta.wikimedia.org/w/index.php?title=Movement_Charter%2FDrafting_Committee%2FNewsletter%2F1&veaction=editsource some time in the past but no longer.

It is correctly degraded to INFO level right now. It's probably one-off case given this error isn't more frequent.

There is one bug in the code, which I'll fix and then we can close this.

Change #1100801 had a related patch set uploaded (by Nikerabbit; author: Nikerabbit):

[mediawiki/extensions/Translate@master] SearchTranslationsSpecialPage: Correct logger placeholder syntax

https://gerrit.wikimedia.org/r/1100801

Change #1100801 merged by jenkins-bot:

[mediawiki/extensions/Translate@master] SearchTranslationsSpecialPage: Correct logger placeholder syntax

https://gerrit.wikimedia.org/r/1100801