In json "null" is a valid value for an item in a list or object. json.loads("null") returns None.
Because next_value returns json.loads(value_string) or None (or a Transient subclass), and that value is compared to None, a list like [1,2,null,3,4,5] is wrongly interpreted.
Here is a comparison with json_stream too.

import adafruit_json_stream
import json_stream
import io

def test_null_item(json_stream):
    print("-"*10)
    source = io.BytesIO(b'[1,2,null,3,4]')
    stream = json_stream.load(source)
    for item in stream:
        print("-", item)

test_null_item(adafruit_json_stream)
test_null_item(json_stream)

----------
- 1
- 2
----------
- 1
- 2
- None
- 3
- 4