diff --git a/pgxn/neon/pagestore_smgr.c b/pgxn/neon/pagestore_smgr.c
index b0712468f5..a7745713cf 100644
--- a/pgxn/neon/pagestore_smgr.c
+++ b/pgxn/neon/pagestore_smgr.c
@@ -681,7 +681,26 @@ prefetch_do_request(PrefetchRequest *slot, bool *force_latest, XLogRecPtr *force
 	Assert(slot->my_ring_index == MyPState->ring_unused);
 	page_server->send((NeonRequest *) &request);
 
-	/* Remember request LSN for the prefetched page to avoid false pefetch invalidations */
+	/*
+	 * Remember request LSN in the last-written LSN cache to avoid false
+	 * prefetch invalidations.
+	 *
+	 * Imagine what would happen without this, when you perform a large
+	 * sequential scan with UPDATE. The sequential scan issues a prefetch
+	 * request for each page in order, and every page is also dirtied. On
+	 * each page, the oldest page in the last-written LSN cache is evicted,
+	 * which advances the global last-written LSN. The pages being scanned are
+	 * not in the last-written cache, so each prefetch request will use the
+	 * global last-written LSN in the request and memorize that in the
+	 * slot. However, when we receive the response to the prefetch request,
+	 * the global last-written LSN has already moved forwards, and the
+	 * cross-check we make that the last-written LSN matches will fail, and we
+	 * discard the prefetched response unnecessary.
+	 *
+	 * Inserting the LSN we use in the prefetch request to the last-written LSN 
+	 * cache avoids that problem. With that, we will use the cached value in
+	 * the cross-check, instead of the more recent global last-written LSN value.
+	 */
 	SetLastWrittenLSNForBlock(
 		request.req.lsn,
 		slot->buftag.rnode,