dict elements are sometimes treated as List and sometimes as String

[buran]

Please, post full traceback you get, in error tags. There is something else, because your code works:
the second code works

words = dict()
token:str = "acto"
words[token] = ["coat"]
words[token].append("taco")
print(words)

Output:
{'acto': ['coat', 'taco']}