I think I have encounted that issue using Python as a Native Messaging host.
The Native Messaging protocol specifies capablility to pass 1MB to the host from client. The hosts exits when 873816 bytes are read from sys.stding.buffer How to read exactly N bytes from stdin in Native Messaging host?.
No. your problem is not relating to stdin buffer at all.
I wrote a reply at another thread.
Please stop posting same issue in multiple thread. I found your post in at least three threads.
In case anyone is thinking about investigating this:
don’t bother, the problem was not with reading from stdin at all, the problem was elsewhere. See thread here.