I (uppercase) with dot above, cause ORA-06502: PL/SQL: numeric or value error: character string buff

We hit error "ORA-06502: PL/SQL: numeric or value error: character string buffer too small" when we do the following:

declare

i number;

v varchar2(30);

v1 varchar2(200);

begin

i:=0;

v1:='AAAAAAA AAİAİA AA AAAAAAAAA AAAAAAAAİ';

dbms_output.put_line(length(v1)||' '||v1);

v:=substr(v1, 1, 28)||' '||to_char(i);

dbms_output.put_line(length(v)||' '||v);

end;

however, if we change the v to varchar2(32), it will be success, but the final length of v is 30.

so what wrong when we declare only  varchar2(30), but the system throw us the error, but in fact it is only 30 characters?