I was not able to reproduce the problem. Maybe this is the timing problem.
Hmm. I can't reproduce this on my Ubuntu box either. I will retry on the Mac machine in the evening. Surprisingly, I could reproduce it very easily on that box. What I'd observed is that the walreceiver on the cascaded standby is stuck at walreceiver.c:447, which in turn is waiting infinitely at libpqwalreceiver.c:501 i.e. PQgetResult() call.
I'll retry and report back if I see the problem on the offending platform.