Aye, the report is definitely appreciated - the interaction between "__main__" execution and normal module imports has always been tricky, so it's always good to be informed of new problems folks encounter.

As an example illustrating the underlying problem here without involving the "-m" switch at all:

>>> from importlib.util import find_spec
>>> find_spec("dis")
ModuleSpec(name='dis', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7fa7ba859fd0>, origin='/usr/lib64/python3.5/dis.py')
>>> import sys
>>> sys.modules["dis"] = object()
>>> find_spec("dis")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/importlib/util.py", line 99, in find_spec
    raise ValueError('{}.__spec__ is not set'.format(name)) from None
ValueError: dis.__spec__ is not set


The difference between 3.5.1 and 3.5.2 is that if you do "find_spec('mypkg.mymod')" *before* "mypkg" is imported, then find_spec will use the original module object rather than the replacement inserted into sys.modules when it recurses down to look for "mypkg.mymod". However, if "mypkg" is already in sys.modules before find_spec is called, then find_spec will attempt to use that existing object, rather than the (potentially no longer available) original module object.