Here's a patch which adds support for the `n` value of the flag. It makes the dbm consistent across implementations and it's thus more reliable.
Also, attached a script which replicates the problem for Windows (where dbm.dumb is used).


My output (after a couple of runs):


D:\Projects\snippets\DBM>py -3 a.py u7
test
[b'u3', b'u2', b'u5', b'u4', b'u7', b'u6']
test.dat exists? True
test.dir exists? True
b'u3' b'there'
b'u2' b'there'
b'u5' b'there'
b'u4' b'there'
b'u7' b'there'
b'u6' b'there'