I wrote:
$ python3 -c 'import sys; sys.stdout.write("foo"); sys.stdout.buffer.write(b"bar"); sys.stdout.write("baz\n")'
barfoobaz

Hmm, the behaviour actually would differ here: the proposed implementation would print

foobaz
bar

(the "foobaz\n" is written by a call to WriteConsoleW and then the "bar" gets flushed to stdout when the process exits).

But since the naive expectation is "foobarbaz\n" and you already have to flush after each call in order to get that, I think this change in behaviour would be unlikely to affect correct applications.