When reading this project's wiki [0], it mentions that Kanal (another channel implementation) uses an optimization that "makes [the] async API not cancellation-safe". I wonder if this is the same / related issue to the recent HN thread on "future lock" [1]. I hadn't heard of this cancellation safety issue prior to that other HN thread.
Futurelock is not about cancellation safety (cancellation is actually one solution to futurelock), though the related issues that are linked in that post are.