ERROR commands out of sync. Did you run multiple statements at once

[letharion]

I'm doing a simple test run locally with gh-ost.

./gh-ost
  --max-load=Threads_running=25
  --critical-load=Threads_running=1000
  --chunk-size=1000
  --max-lag-millis=1500
  --user=<user>
  --password=<password>
  --host=localhost
  --database=<db-name>
  --table=<table-name>
  --allow-on-master
  --verbose '--alter=ADD COLUMN test_column INT' 
  --switch-to-rbr
  --cut-over=default
  --exact-rowcount
  --concurrent-rowcount
  --default-retries=120
  --panic-flag-file=/tmp/ghost.panic.flag
  --postpone-cut-over-flag-file=/tmp/ghost.postpone.flag
  --execute

This appears to work fine, but I get these error messages:

Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m20s(total), 23s(copy); streamer: mysql-bin.000001:43837497; Lag: 0.15s, State: postponing cut-over; ETA: due
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m25s(total), 23s(copy); streamer: mysql-bin.000001:43863861; Lag: 0.11s, State: postponing cut-over; ETA: due
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m30s(total), 23s(copy); streamer: mysql-bin.000001:43889675; Lag: 0.17s, State: postponing cut-over; ETA: due
2021-04-16 11:14:24 ERROR commands out of sync. Did you run multiple statements at once?
2021-04-16 11:14:24 ERROR commands out of sync. Did you run multiple statements at once?
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m35s(total), 23s(copy); streamer: mysql-bin.000001:43906779; Lag: 1.51s, State: postponing cut-over; ETA: due
2021-04-16 11:14:26 ERROR commands out of sync. Did you run multiple statements at once?
2021-04-16 11:14:26 ERROR commands out of sync. Did you run multiple statements at once?
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m40s(total), 23s(copy); streamer: mysql-bin.000001:43906779; Lag: 1.51s, State: postponing cut-over; ETA: due
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m45s(total), 23s(copy); streamer: mysql-bin.000001:43906779; Lag: 1.51s, State: postponing cut-over; ETA: due
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 2m55s(total), 23s(copy); streamer: mysql-bin.000001:43934664; Lag: 0.07s, State: postponing cut-over; ETA: due
Copy: 100000/100000 100.0%; Applied: 1; Backlog: 0/1000; Time: 3m0s(total), 23s(copy); streamer: mysql-bin.000001:43962657; Lag: 0.07s, State: postponing cut-over; ETA: due

Now what's odd with this is that there's almost no activity on my local environment, so I have no idea what statements this would referring to? I was running some selects again the _gho table, but that's it.

I couldn't find any other issues which lists this error, so I didn't find any other clues either.