See discussion at [e70481a4]. Snippet:
Standard INITIALIZE never changes filler items
Quoted from ANSI INCITS 23-1985:
Index data items and elementary FILLER data items are not affected by the execution of an INITIALIZE statement.
Where 2002 adds the [WITH] FILLER phrase
the following data items are excluded as receiving-operands [and are therefore unchanged]: [...] If the FILLER phrase is not specified, elementary data items with an explicit or implicit FILLER clause
Therefore GNU Cobol is standard-conforming (and seem to work as MicroFocus and ACU-COBOL ever did, current ILE COBOL and Enterprise COBOL for z/OS docs state the same).
If there are other vendors initializing all fillers by default we may should come up with a conf entry.