Wrong query results on column ORA_ROWSCN with bind variables and NVL (Oracle 12c)
The plan clearly shows the OR-expansion query transformation. It is not the first time it produced wrong results due to various bugs.

Try running your query as:

SELECT /*+ NO_EXPAND */
  rid, ora_rowscn
FROM
  my_table
WHERE
  rid = NVL(:myId, rid);


Or set the below parameters before executing it:

alter session set "_no_or_expansion"=true;
alter session set "_or_expand_nvl_predicate"=false;


Then run your query.