From the given example, if I add "await q.aclose()" after "await q.asend(123456)" it does not leak the memory.

This is a good example showing that we should always wrap async generators with explicit "aclosing" context manager (which does not exist yet in the stdlib).
I'm already doing so by writing a custom library:
https://github.com/achimnol/aiotools/blob/ef7bf0ce/src/aiotools/context.py#L152

We may need to update the documentation to recommend explicit aclosing of async generators.