Hello Swift Community,
The Core Team has accepted an amendment to SE-0298 that clarifies the end-of-iteration behavior:
End of Iteration
After an AsyncIteratorProtocol types returns nil or throws an error from its next() method, all future calls to next() must return nil. This matches the behavior of IteratorProtocol types and is important, since calling an iterator's next() method is the only way to determine whether iteration has finished.
Doug