diff --git a/includes/libs/rdbms/database/Database.php b/includes/libs/rdbms/database/Database.php
index 699fee7e74f..8739cbf706c 100644
--- a/includes/libs/rdbms/database/Database.php
+++ b/includes/libs/rdbms/database/Database.php
@@ -3393,6 +3393,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
 		$this->trxWriteCallers = [];
 		// First SELECT after BEGIN will establish the snapshot in REPEATABLE-READ.
 		// Get an estimate of the replication lag before any such queries.
+		$this->trxReplicaLag = null; // clear cached value first
 		$this->trxReplicaLag = $this->getApproximateLagStatus()['lag'];
 		// T147697: make explicitTrxActive() return true until begin() finishes. This way, no
 		// caller will think its OK to muck around with the transaction just because startAtomic()
@@ -3666,7 +3667,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
 	}
 
 	public function getSessionLagStatus() {
-		return $this->getTransactionLagStatus() ?: $this->getApproximateLagStatus();
+		return $this->getRecordedTransactionLagStatus() ?: $this->getApproximateLagStatus();
 	}
 
 	/**
@@ -3677,11 +3678,13 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
 	 * is this lag plus transaction duration. If they don't, it is still
 	 * safe to be pessimistic. This returns null if there is no transaction.
 	 *
+	 * This returns null if the lag status for this transaction was not yet recorded.
+	 *
 	 * @return array|null ('lag': seconds or false on error, 'since': UNIX timestamp of BEGIN)
 	 * @since 1.27
 	 */
-	final protected function getTransactionLagStatus() {
-		return $this->trxLevel
+	final protected function getRecordedTransactionLagStatus() {
+		return ( $this->trxLevel && $this->trxReplicaLag !== null )
 			? [ 'lag' => $this->trxReplicaLag, 'since' => $this->trxTimestamp() ]
 			: null;
 	}
diff --git a/includes/libs/rdbms/database/DatabaseMysqlBase.php b/includes/libs/rdbms/database/DatabaseMysqlBase.php
index 0a63bdc6f10..86ec641668b 100644
--- a/includes/libs/rdbms/database/DatabaseMysqlBase.php
+++ b/includes/libs/rdbms/database/DatabaseMysqlBase.php
@@ -766,18 +766,20 @@ abstract class DatabaseMysqlBase extends Database {
 	protected function getLagFromPtHeartbeat() {
 		$options = $this->lagDetectionOptions;
 
-		$staleness = $this->trxLevel
-			? microtime( true ) - $this->trxTimestamp()
-			: 0;
-		if ( $staleness > self::LAG_STALE_WARN_THRESHOLD ) {
-			// Avoid returning higher and higher lag value due to snapshot age
-			// given that the isolation level will typically be REPEATABLE-READ
-			$this->queryLogger->warning(
-				"Using cached lag value for {db_server} due to active transaction",
-				$this->getLogContext( [ 'method' => __METHOD__ ] )
-			);
+		$currentTrxInfo = $this->getRecordedTransactionLagStatus();
+		if ( $currentTrxInfo ) {
+			// There is an active transaction and the initial lag was already queried
+			$staleness = microtime( true ) - $currentTrxInfo['since'];
+			if ( $staleness > self::LAG_STALE_WARN_THRESHOLD ) {
+				// Avoid returning higher and higher lag value due to snapshot age
+				// given that the isolation level will typically be REPEATABLE-READ
+				$this->queryLogger->warning(
+					"Using cached lag value for {db_server} due to active transaction",
+					$this->getLogContext( [ 'method' => __METHOD__, 'age' => $staleness ] )
+				);
+			}
 
-			return $this->getTransactionLagStatus()['lag'];
+			return $currentTrxInfo['lag'];
 		}
 
 		if ( isset( $options['conds'] ) ) {