with JSON data, index is not used

Hi
I have the following table :
CREATE TABLE EIGJSON.MET_P6_REF_POINTB2
(
ELEMENT CLOB
)
LOB (ELEMENT) STORE AS SECUREFILE (
TABLESPACE TOOLS
ENABLE STORAGE IN ROW
CHUNK 8192
NOCACHE
LOGGING...
and this index :

CREATE INDEX EIGJSON.IDX_MET_P6_REF_POINTB_01 ON EIGJSON.MET_P6_REF_POINTB2
(JSON_VALUE("ELEMENT" FORMAT JSON , '$.DATE_PUBLI' RETURNING VARCHAR2(4000) ERROR ON ERROR), JSON_VALUE("ELEMENT" FORMAT JSON , '$.TYPE_PREV' RETURNING NUMBER ERROR ON ERROR), JSON_VALUE("ELEMENT" FORMAT JSON , '$.LOCALISATION' RETURNING NUMBER ERROR ON ERROR), JSON_VALUE("ELEMENT" FORMAT JSON , '$.DATA.DATE_VALID' RETURNING VARCHAR2(4000) ERROR ON ERROR))
LOGGING
TABLESPACE IDX_USERS