Redirect Loop issue on one mapping (others work fine)

I seem to be having the same issue as brought up in this thread:
https://www.ads-software.com/support/topic/plugin-wordpress-mu-domain-mapping-infinite-loop-problem?replies=14

That issue was marked as resolved, but there isn’t a definitive (working) answer…

I have Multisite set to subdomains. I’ve got many sites with successful domain mappings. However, one of our recent domain mappings results in a redirect loop.

The A Record is set correctly, and working. When ‘Primary Domain’ is UNCHECKED, entering mapped domain in the browser (correctly) points to subsite.mainsite.com.

But when I set ‘Primary Domain’ to CHECKED, entering the mapped domain in the browser results in a redirect loop.

As Andrea responded here: https://www.ads-software.com/support/topic/plugin-wordpress-mu-domain-mapping-infinite-loop-problem?replies=14#post-1955097 … Saying the fix is to edit that site and change all instances of subsite.mainsite.com to the mapped domain.

I tried that, but then, entering the mapped domain in the browser redirects to the mainsite.com (main site of the network).

Is there another solution to this? It’s strange that the problem exists only on this domain mapping and not the others (which are all working fine).

https://www.ads-software.com/extend/plugins/wordpress-mu-domain-mapping/