It seems highly likely this is related to #26110 which optimizes method calls by avoiding the creation of bound methods in certain circumstances, and/or the follow-up #29263.

Side-note:

bound_method = functools.partial(method, self)

is not how you create real bound methods. The correct approach (that makes a bound method identical to what the interpreter makes when necessary) for your use case is:

bound_method = types.MethodType(method, self)