\IPS\Content\Search\MySql::resetLastComment you do

			$latest = \IPS\Db::i()->select( 'index_object_id, index_date_updated, index_class', 'core_search_index', array( \IPS\Db::i()->in( 'index_class', $classes ) . ' AND index_item_id=? and index_hidden=0', $indexItemId ), 'index_date_created DESC', array( 0, 1 ) )->first();

Followed later by

		\IPS\Db::i()->update( 'core_search_index', array( 'index_date_updated' => $latest['index_date_updated'], 'index_date_commented' => $latest['index_date_commented'] ), array( \IPS\Db::i()->in( 'index_class', $classes ) . ' AND index_item_id=? AND index_object_id=?', $indexItemId, ( $firstCommentId ?? $indexItemId ) ) );

but index_date_commented was not loaded in the query, so this line fails IN_DEV with an array index not found exception.

Could you please confirm you are using the latest release there?

Yes, I verified that this bug exists in 107801

Thank you for bringing this issue to our attention! I can confirm this should be further reviewed and I have logged an internal bug report for our development team to investigate and address as necessary, in a future maintenance release.