BUG #15637: Problem insert data 【Japanese】

The following bug has been logged on the website:

Bug reference:      15637
Logged by:          Thanh Huynh Van
Email address:      thanhhv26@gmail.com
PostgreSQL version: 11.1
Operating system:   Window Server 2012 Standard
Description:

I created table has a column　【"C_VAL" character varying(10)】　after that I
insert data【'不具合管理報告書': length=24】. It was inserted successfully => this is
problem.
* Details:
-- Step 1: Create table. Data length of "C_VAL" column = 10
create table "USER02"."T_331_VARCHAR" (
  "ID" numeric not null
  , "C_VAL" character varying(10)
  , primary key ("ID")
);
-- Step 2: Check data length. OCTET_LENGTH('不具合管理報告書') = 24
SELECT OCTET_LENGTH('不具合管理報告書');
-- Step 3: Insert data. Length '不具合管理報告書' = 24
INSERT INTO "USER02"."T_331_VARCHAR"("ID", "C_VAL")
VALUES(1,'不具合管理報告書');
-- Step 4: Check length of inserted data
SELECT OCTET_LENGTH("C_VAL"), * FROM "USER02"."T_331_VARCHAR";
=> Why 【Step 3】was successful ?. Can you help me ?
Thank you !