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We have a special case here.
Basically, there are two ways to call write() now:

1. Legacy writer.write(b'data')
2. Recommended await writer.write(b'data')

For the second case, an exception raised by nested writer.drain() call is propagated to user.
The first case ignores the exception from nested writer.drain() call, allowing existing code to work in a backward compatible way.
A subsequent call to drain() raises the exception again, like in the following snippet:

writer.write(b'data')  # exception is swallowed
await writer.drain()  # previous exception is raised if any

Sorry for such a tricky code.
Ideally, we should make writer.write() a coroutine from the very beginning.
Proposed workaround is a strategy to keep backward compatibility and provide a smooth transition path.