Forwarding your question to Beda, who owns this package (I am assuming you're using GET_KEYS on JSON_OBJECT_T, right?).
He should reply shortly...
Just got access to your Google drive files, and looks like you're not using the Oracle Database
PLSQL JSON package:
So can't help you there, sorry. I thought you were asking about the Oracle Database PLSQL JSON package,
which is owned by our team. Not knowledgeable about the package you're using.
If you can't resole the issue, you might want to consider switching to the Oracle Database
PLSQL JSON package linked above. You'd need Oracle Database 12.2 (or above) though.