iis 10 strange behavior http vs. https - php flush - responseBufferLimit is 0

I am using FastCGI to run PHP7 on an IIS10 webserver for two sites:

1. Default Web Site binded to port 80 and port 443
2. Development (port 5588)

both of which have responseBufferLimit set to 0. The first site also has ssl enabled. Outputting long php scripts before they end, works on site 2 http and site 1 https, but not site 1 http.

For testing purposes I am using:

<?php
set_time_limit(0);

function flush_buffers()
{
    ob_end_flush();
    ob_flush();
    flush();
    ob_start();
}

ob_start();
flush_buffers();
echo "starting...<br/>\n";
for($i = 0; $i < 2; $i++) 
{
    flush_buffers();
    print   "$i<br/>\n";
    flush_buffers();
    sleep(1);
}

flush_buffers();

print "DONE!<br/>\n";
?>

Also, if I create another site, with the same PHP sources, on another port, it also works. But if I try to use the same 80 port on the new site over http, it stops working.

• I tried setting responseBufferLimit over in C:\inetpub\wwwroot\web.config and in C:\Windows\System32\inetsrv\config\applicationHost.config.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
...
<handlers>
<remove name="FastCGI" />
<add name="FastCGI" path="*.php" verb="*" type="" modules="FastCgiModule" scriptProcessor="E:\PHP7\php-cgi.exe" resourceType="File" requireAccess="Script" allowPathInfo="false" preCondition="" responseBufferLimit="0" />
</handlers>
...
</system.webServer>
</configuration>

• iisreset after every modification
• removed my FastCGI module then placed it back
• did everything through the IIS Manager GUI too
• tried adding with CLI too. FastCGI is my handler name ofc.

appcmd.exe set config "Default Web Site" -section:system.webServer/handlers /[name='FastCGI'].responseBufferLimit:"0"

• tried setting it to 1 instead of 0
• put it back to 0
• iisreset after each mod ofc
• I even removed the ssl bind from 1st site. Same behavior
• Tried setting dynamic compression to false too

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <urlCompression doDynamicCompression="false" />
    </system.webServer>
</configuration>

I am purely lost