A mutable list or dict seems to be used as default parameter value:

test/functional/test_framework/psbt.py:    def __init__(self, g=PSBTMap(), i=[], o=[]):

This is how mutable list and dict default parameter values behave:

>>> def f(i, j=[], k={}):
...     j.append(i)
...     k[i] = True
...     return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1, 1], {1: True})
>>> f(2)
([1, 1, 2], {1: True, 2: True})

The intended behaviour was likely:

>>> def f(i, j=None, k=None):
...     if j is None:
...         j = []
...     if k is None:
...         k = {}
...     j.append(i)
...     k[i] = True
...     return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1], {1: True})
>>> f(2)
([2], {2: True})