Domain mapping redirects to main domain instead of subdirectory

I'm having the same mapping issue described in a couple of other threads but can't see the problem.

Our setup:
* hosts the WPMS (subfolder) on a dedicated IP at the load balancer. When you browse to the IP the main site shows fine.
* We wish to map to display the same as
* Put in Domain Mapping version per the instructions (/wp-content/plugins/domain-mapping/).
* Moved sunrise.php to the wp-content and made sure the original is gone.
* Put define( 'SUNRISE', 'on' ); in the proper place in wp-config.php
* Network activated it fine, set the server IP address as recommended.
* Check that the target subfolder site displays fine at
* In the target subfolder site, mapped domain Front end redirects should be: 'Disabled and entered domain should be used". After saving, the page looks fine, Health status = Valid.
* Network admin > Domain mapping > Mapped domains looks fine:
** SiteID=513
** Mapped domain=
** Original address=
** Health=valid
** DNS Configuration=Host Name:; Record Type: A; Value: <the correct IP>
** Active=yes
* I've reset the .htaccess to the default multisite config per
* I've moved all other plugin/* folders off eslewhere to disable them.
* I've disabled the plugin and re-enabled it.
* I've deleted the mappings and tried again on another subfolder site.
* Nothing relevant shows in the PHP error logs or wp-content/debug.log
* The same thing happens if I set "Front end redirect should be:" to "Directed to mapped (primary) domain" or "Directed to original domain".
* I'm testing in a fresh incognito browser for each test.
* Restarting Apache and PHP-FPM had no effect.

The problem is that redirects to the main site

200 remains available (which is OK) redirects to

Can you guys spot what I'm missing?