Migrating 100%, but shows State: postponing cut-over; ETA: due

Hi, I'm new to this. I try to use this greate tool for modifying table structure. When migrating completed 100%, but continuously pop up:

> Copy: 22102848/22102848 100.0%; Applied: 110; Backlog: 0/1000; Time: 3h22m0s(total), 3h6m12s(copy); streamer: mysql-bin.000018:912494008; State: postponing cut-over; ETA: due

and my command is this:

> ./gh-ost \
--max-load=Threads_running=25 \
--critical-load=Threads_running=1000 \
--chunk-size=1000 \
--max-lag-millis=1500 \
--user="xxxx" \
--password="xxxx" \
--host=xxxx \
--port=4223 \
--database="test" \
--table="t1" \
--verbose \
--alter="ADD COLUMN isBookmarkedThree TINYINT(1) NOT NULL DEFAULT 0" \
--allow-master-master \
--cut-over=default \
--exact-rowcount \
--concurrent-rowcount \
--default-retries=120 \
--panic-flag-file=./ghost.panic.flag \
--postpone-cut-over-flag-file=./ghost.postpone.flag \
-aliyun-rds \
--allow-on-master \
--execute

I try to delete ghost.postpone.flag file, it kept popping up:

> 2019-12-06 19:25:39 INFO Grabbing voluntary lock: gh-ost.319743.lock
2019-12-06 19:25:39 INFO Setting LOCK timeout as 6 seconds
2019-12-06 19:25:39 INFO Looking for magic cut-over table
2019-12-06 19:25:39 INFO Creating magic cut-over table `test`.`_t1_del`
2019-12-06 19:25:39 INFO Magic cut-over table created
2019-12-06 19:25:39 INFO Locking `test`.`t1`, `test`.`_t1_del`
2019-12-06 19:25:39 INFO Tables locked
2019-12-06 19:25:39 INFO Session locking original & magic tables is 319743
2019-12-06 19:25:39 INFO Writing changelog state: AllEventsUpToLockProcessed:1575631539838703726
2019-12-06 19:25:39 INFO Waiting for events up to lock
2019-12-06 19:25:42 ERROR Timeout while waiting for events up to lock
2019-12-06 19:25:42 ERROR 2019-12-06 19:25:42 ERROR Timeout while waiting for events up to lock
2019-12-06 19:25:42 INFO Looking for magic cut-over table
2019-12-06 19:25:42 INFO Will now proceed to drop magic table and unlock tables
2019-12-06 19:25:42 INFO Dropping magic cut-over table
2019-12-06 19:25:42 INFO Releasing lock from `test`.`t1`, `test`.`_t1_del`
2019-12-06 19:25:42 INFO Tables unlocked

serveral minutes later, ends like:

> 2019-12-06 19:27:39 ERROR Timeout while waiting for events up to lock
2019-12-06 19:27:39 ERROR 2019-12-06 19:27:39 ERROR Timeout while waiting for events up to lock
2019-12-06 19:27:39 INFO Looking for magic cut-over table
2019-12-06 19:27:39 INFO Will now proceed to drop magic table and unlock tables
2019-12-06 19:27:39 INFO Dropping magic cut-over table
2019-12-06 19:27:39 INFO Removing socket file: /tmp/gh-ost.test.t1.sock
2019-12-06 19:27:39 FATAL 2019-12-06 19:27:39 ERROR Timeout while waiting for events up to lock

Is there anything I missed? Thanks