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:
* blogs.somedomain.com 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 somesite.somedomain.com to display the same as blogs.somedomain.com/somesite
* Put in Domain Mapping version 4.4.2.4 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 https://blogs.somedomain.com/somesite
* In the target subfolder site, mapped domain https://somesite.somedomain.com. 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=https://somesite.somedomain.com
** Original address=https://blogs.somedomain.com/somesite
** Health=valid
** DNS Configuration=Host Name: blogs.somedomain.com; Record Type: A; Value: <the correct IP>
** Active=yes
* I've reset the .htaccess to the default multisite config per https://codex.wordpress.org/htaccess#Multisite
* 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 https://somesite.somedomain.com redirects to the main site https://blogs.somedomain.com/:

302 http://somesite.somedomain.com/
302 https://somesite.somedomain.com/
200 https://blogs.somedomain.com/

https://blogs.somedomain.com/somesite/ remains available (which is OK)
https://somesite.somedomain.com/somesite/ redirects to https://blogs.somedomain.com/somesite/

Can you guys spot what I'm missing?