Graphlib Topological sorting not showing vertex that is not completed

Question

topological_sorter.get_ready() does not seem to show the same vertex twice even if it was not yet marked as done using topological_sorter.done(node)

from graphlib import TopologicalSorter

graph = {"D": {"B", "C"}, "C": {"A"}, "B": {"A"}, "E": {}}
topological_sorter = TopologicalSorter(graph)
topological_sorter.prepare()

def return_active():
    if topological_sorter.is_active():
        active_list = []
        for node in topological_sorter.get_ready():
            active_list.append(node)
    return active_list

a = return_active()
print(a)

topological_sorter.done("A")

b = return_active()
print(b)

Output:

a = ['A', 'E']

b = ['C', 'B']

I was expecting b = ['E', 'C', 'B'] instead of ['C', 'B'] only