PXB-3762: Backup with --transition-key crashes on assertion failure in xb_set_encryption() when undo log encryption keys exist in redo log

storage/innobase/srv/srv0start.cc

@@ -392,7 +392,7 @@ static dberr_t srv_undo_tablespace_read_encryption(pfs_os_file_t fh,
 
   if (found && recv_key->lsn >= page_lsn) {
     // Condition 1: Use key from redo if available and appropriate
-    encryption_success = use_dumped_tablespace_keys
+    encryption_success = (use_dumped_tablespace_keys && !srv_backup_mode)
                              ? load_key_from_dump()
                              : set_encryption(recv_key->ptr, recv_key->iv);
   } else if (is_enc) {

storage/innobase/xtrabackup/test/suites/keyring/pxb-3762-transition-key-undo-encrypt-crash.sh

@@ -0,0 +1,37 @@
+#
+# Stable reproduction: assertion failure in xb_set_encryption() during
+# backup with --transition-key when undo encryption keys exist in redo log.
+#
+# Strategy:
+#   1. Force a checkpoint to establish a clean baseline on disk.
+#   2. Freeze page cleaner and checkpoint so the undo page stays unflushed.
+#   3. Rotate master key — writes a new undo encryption key entry to redo log
+#      while the on-disk undo page retains the pre-rotation LSN.
+#   4. Start backup with --transition-key (sets use_dumped_tablespace_keys=true).
+#
+#   This guarantees recv_find_encryption_key() returns found=true with
+#   recv_key->lsn > page_lsn, entering the buggy Condition 1 branch.
+#
+
+require_server_version_higher_than 8.4.0
+require_debug_server
+
+KEYRING_TYPE="component"
+. inc/keyring_common.sh
+. inc/keyring_file.sh
+configure_server_with_component
+
+mysql -e "CREATE TABLE t (a INT) ENCRYPTION='y'" test
+mysql -e "INSERT INTO t VALUES (1)" test
+
+# Force checkpoint, then freeze page cleaner and checkpoint.
+# Rotate master key AFTER freezing so the undo page is not flushed to disk.
+mysql test <<EOF
+SET GLOBAL innodb_log_checkpoint_now = 1;
+SET GLOBAL innodb_page_cleaner_disabled_debug = 1;
+SET GLOBAL innodb_checkpoint_disabled = 1;
+ALTER INSTANCE ROTATE INNODB MASTER KEY;
+EOF
+
+xtrabackup --backup --transition-key=123 --target-dir=$topdir/backup
+xtrabackup --prepare --transition-key=123 --target-dir=$topdir/backup