Looking over the changelog, my guess (untested) is this is caused by commit d6a283b3 for Issue 25862. That change looks like it drops the internal TextIOWrapper decoding buffer for each successful write.

I don't have the right version of Python to test with, but I expect this to also be broken without using a socket:

>>> f = TextIOWrapper(BufferedRWPair(BytesIO(b"Hello\nYou\n"), BytesIO()))
>>> f.readline()
'Hello\n'
>>> f.write(_)
6
>>> f.readline()  # Does this now return EOF?
'You\n'