% python3.6 -c "import io; b=io.BytesIO(); assert b.truncate(42) == 42; assert len(b.getbuffer()) == 42, 'expected length 42, got {}'.format(len(b.getbuffer()))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AssertionError: expected length 42, got 0

The documentation [0] says that truncate can extend objects, and there is no indication that BytesIO does not support that.  As demonstrated, truncate returns the new size, but the buffer obtained from that BytesIO is of size zero (likewise, b.getvalue() returns b''.

0: https://docs.python.org/3.6/library/io.html#io.IOBase.truncate