A result of modifying the allocator slightly in WT-9950 is that the JSON cursor code broke. See the commented out scenarios in test_jsondump01.py . The old code may be relying on realloced memory being completely cleared. In _wt_json_alloc_unpack, after the call to _wt_realloc_noclear, a fix of doing a memset of 0 may suffice, but we should understand why it needs to be cleared.