I have no idea what the problem could be.
My only experience would be the time I had to deal with Greek characters.
The solution that worked for me: ensure everything was utf8.
I think it may be a font rendering problem.
Can you look at the json with some other program?
If you load that JSON into the DB, are the characters correct?
Use a HEX editor to take a look at the binary values for the JSON string.
Within the database, for the source row, do a DUMP() of the specific column.
select dump( bank ) from T where account_day=27
find the NLS_LANG value of the database.
take a look at the HTTP header. (i think the value you are looking for is enctype)