Oracle 19 ORA-01450: maximum key length (6398) exceeded

I am using Oracle 19 and recently changed the collation to XGERMAN_AI. This required to change the MAX_STRING_SIZE to be EXTENDED as in the following link https://docs.oracle.com/database/121/REFRN/GUID-D424D23B-0933-425F-BC69-9C0E6724693C.htm#REFRN10321

Now, trying to create tables with unique constraints on varchar2 columns produce the error

ORA-01450: maximum key length (6398) exceeded.

Here is my table

CREATE TABLE MY_TABLE (     ID NUMBER(38, 0) DEFAULT PKS_MY_TABLE_SEQ.nextval NOT NULL,      VERSION NUMBER(38, 0) DEFAULT 0 NOT NULL,      CREATED_BY VARCHAR2(50 CHAR) NOT NULL,      CREATED_AT TIMESTAMP NOT NULL,      MODIFIED_BY VARCHAR2(50 CHAR) NOT NULL,      MODIFIED_AT TIMESTAMP NOT NULL,      ID_OLD NUMBER(38, 0),      RISK_LEVEL VARCHAR2(255 CHAR) NOT NULL,      REMARKS VARCHAR2(255 CHAR),      IS_DEACTIVATED NUMBER(1) DEFAULT 0 NOT NULL,      COLOR VARCHAR2(255 CHAR) NOT NULL,      CONSTRAINT MY_TABLE_PK PRIMARY KEY (ID),      UNIQUE (RISK_LEVEL) ) 

I googled this error, and all results say that it is becase the index size is bigger than the block size which is 8k. But in my case the column RISK_LEVEL is only 255 char long! (255*4 bytes = 1020 bytes), which is much less than 8k and also 6398

Any idea how to fix this?