9.6.11- could not truncate directory "pg_serial": apparent wraparound

Hi,

PG: 9.6.11
OS: CentOS
Env: AWS EC2

I've faced the following exceptions in PostgreSQL server log:
> could not truncate directory "pg_serial": apparent wraparound
Sometimes it repeats every 5 min and the longest period was 40 min.

In fact, I can't find any suspicious events happening that periods.
pg_wait_sampling didn't catch any events, no long queries (more than 60s),
Autovacuum workers or transactions in "idle in transaction" state were in
action at this time.

The only related I could find in archive is:
https://www.postgresql.org/message-id/CACjxUsON4Vya3a6r%3DubwmN-4qTDDfZjuwSzjnL1QjdUc8_gzLw%40mail.gmail.com
>You should not see the errors you are reporting nor
>the warning I mentioned unless a serializable transaction remains
>active long enough for about 1 billion transaction IDs to be
>consumed.

Database age now is just 18.5 millions of transactions.

Server has two standbys (sync and async), hot_standby_feedback is off.

Please advice what I can do to find a reason of these exceptions.