Is there a bug with PL/Scope in combination with associative arrays?
It is a bug, because the documentation states:


  USAGE_CONTEXT_ID is a reflexive foreign key to USAGE_ID


UPDATE: A bit of troubleshooting...

all_identifiers.usage_id comes from sys.plscope_action$.action#

Given:

SQL&gt; /

IDENTIFIER   USAGE       TYPE                 USAGE_ID USAGE_CONTEXT_ID LINE COL
------------ ----------- ------------------ ---------- ---------------- ---- ---
TQ84_PKG_C   DECLARATION PACKAGE                     1                0    1   9
PROC_1       DECLARATION PROCEDURE                   2                1    2  15
TQ84_PKG_C   DEFINITION  PACKAGE                     1                0    1  14
NUM_T        DECLARATION ASSOCIATIVE ARRAY           2                1    3  10
NUMBER       REFERENCE   NUMBER DATATYPE             3                2    3  28
VARCHAR2     REFERENCE   CHARACTER DATATYPE          4                3    3  44
PROC_2       DECLARATION PROCEDURE                   5                1    5  15
PROC_2       DEFINITION  PROCEDURE                   6                5    5  15
PROC_1       DEFINITION  PROCEDURE                   7                1    9  15
V_NUM        DECLARATION VARIABLE                    8                7   10   9
NUM_T        REFERENCE   ASSOCIATIVE ARRAY           9                8   10  15
V_NUM        REFERENCE   VARIABLE                   11               10   12  12
PROC_2       CALL        PROCEDURE                  12               10   13  12

13 rows selected.

SQL&gt;


... we're looking for the elusive sys.plscope_action$.action# = 10 for this object:

SQL&gt; select a.obj#, a.action#, a.signature, a.action, a.line, a.col, a.context#, o.name, o.type#
from sys.plscope_action$ a, sys.obj$ o
where a.obj# = o.obj#
and name = 'TQ84_PKG_C'
and action# = 10;

      OBJ#    ACTION# SIGNATURE                            ACTION LINE COL   CONTEXT# NAME                                TYPE#
---------- ---------- -------------------------------- ---------- ---- --- ---------- ------------------------------ ----------
     83504         10 0C679E5FFBC26144A7F875495F68C2E1          3   12  18          7 TQ84_PKG_C                             11

SQL&gt;


That's the missing row.

The ALL_IDENTIFIERS view joins sys.plscope_identifier$ with sys.plscope_action$ on the signature column - there's no matching row in sys.plscope_identifier$, which I suspect is causing the problem.