Return value error of‘to_timestamp’

When the fractional seconds more than 6, the return value of to_timestamp without truncation.
Postgres think the total value of the.123456789 as a fractional second part and convert to microsecond stored in database.
example:
postgres=# select to_timestamp('1990-1-1 11:11:11.123456789', 'YYYY-MM-DD HH24:MI:SS.US');
to_timestamp
-------------------------------
1990-01-01 11:13:14.456789+08
(1 row)
postgres=#