curl not sending keep-alive packets

[Cary Lewis]

I downloaded and compiled curl on cygwin, version 3.1.5-1 using:

./configure --disable-shared --enable-debug --prefix=/usr

The build completed correctly.

When I attempt to use curl to access an https:// REST endpoint that takes
up to 5 minutes to respond with the:

--keepalive-time 5

command line option, and I watch the TCP traffic via wireshark, I do not
see any TCP Keep-Alive packets. Without the keep-alive packets the request
fails.

curl/libcurl version
curl 7.71.0-DEV (x86_64-pc-cygwin) libcurl/7.71.0-DEV OpenSSL/1.1.1f
zlib/1.2.11
Release-Date: [unreleased]

operating system
CYGWIN_NT-10.0 IT008 3.1.4(0.340/5/3) 2020-02-19 08:49 x86_64 Cygwin

Running the win32 version of curl does work as expected, and I see the
Keep-Alive packets being sent every 5 seconds, and the REST call does
complete correctly.

The official curl build does not work any better, that's why I tried to
download and rebuild the code.

Does anyone know why the --keepalive-time option is not working with cygwin?