One of the domain’s I mapped 301 redirects to main homepage domain

hy ??
usually these issues come from cached pages or redirects. but since you are experienced with the plugin, i guess you have disabled and flushed your browsers cache or tried a fresh browser already?

Yes I don’t even have cache enabled for this site and all testing has been done with browser cache refreshed each time. It occurs in any browser, in private mode, and on other devices.

I can’t really imagine what is causing it for just this one mapped domain, when all the other mapped domains I have done are working fine.

hy.
you can try solution 2 from this source: https://maheshwaghmare.com/avoid-wordpress-canonical-redirection/

although we have already taken care of the canonical redirect in our plugin, this could have something to do with it…

Putting that solution in my functions.php doesn’t seem to change anything.

The issue seems to be with the domain itself rather than the page, because if I map a different domain to the page the page loads fine. But there’s nothing special about the domain at all, it just points to my server like the rest of them with an A Record to the server IP.

hy,
i found this plugin with a quick search (not tested yet): https://www.ads-software.com/plugins/debug-wp-redirect/

it could give you some information where and why this redirect happens

Ah that plugin looks interesting. I tried it out and got this:

Debug WP Redirect
Location
https://themain.com/
Status
301
User Logged In
No
Backtrace
?	Level 1
o	File: /home/appuser/webapps/website/wp-includes/canonical.php
o	Line: #799
o	Function: wp_redirect
o	Function Arguments:
o	array(2) {
o	  [0]=>
o	  string(28) "https://themain.com/"
o	  [1]=>
o	  int(301)
o	}
?	Level 2
o	File: /home/appuser/webapps/website/wp-includes/class-wp-hook.php
o	Line: #307
o	Function: redirect_canonical
o	Function Arguments:
o	array(1) {
o	  [0]=>
o	  string(28) "https://mapped.com/"
o	}
?	Level 3
o	File: /home/appuser/webapps/website/wp-includes/class-wp-hook.php
o	Line: #331
o	Class: WP_Hook
o	Object: WP_Hook
o	Type: ->
o	Function: apply_filters
o	Function Arguments:
o	array(2) {
o	  [0]=>
o	  NULL
o	  [1]=>
o	  array(1) {
o	    [0]=>
o	    string(0) ""
o	  }
o	}
?	Level 4
o	File: /home/appuser/webapps/website/wp-includes/plugin.php
o	Line: #474
o	Class: WP_Hook
o	Object: WP_Hook
o	Type: ->
o	Function: do_action
o	Function Arguments:
o	array(1) {
o	  [0]=>
o	  array(1) {
o	    [0]=>
o	    string(0) ""
o	  }
o	}
?	Level 5
o	File: /home/appuser/webapps/website/wp-includes/template-loader.php
o	Line: #13
o	Function: do_action
o	Function Arguments:
o	array(1) {
o	  [0]=>
o	  string(17) "template_redirect"
o	}
?	Level 6
o	File: /home/appuser/webapps/website/wp-blog-header.php
o	Line: #19
o	Function: require_once
o	Function Arguments:
o	array(1) {
o	  [0]=>
o	  string(68) "/home/appuser/webapps/website/wp-includes/template-loader.php"
o	}
?	Level 7
o	File: /home/appuser/webapps/website/index.php
o	Line: #17
o	Function: require
o	Function Arguments:
o	array(1) {
o	  [0]=>
o	  string(55) "/home/appuser/webapps/website/wp-blog-header.php"
}

hm. and does this output change if you apply solutions #1 / #2 from my previous link?
from the stack i can see that indeed canonical redirect is happending …

When I add

add_filter( 'redirect_canonical', '__return_false' );

to my functions.php then the debug doesn’t come up at all, it just redirects mapped.com to themain.com as if the debug plugin wasn’t even activated. When I remove that code from functions.php the debug appears again when you try to visit mapped.com.

thats interesting… unfortunately we cant offer on-page-support for more detailed review. but i guess you’ll be able to find php devs / freelancer who can continue diving into the redirects and filters.

i’m sorry that i cant offer a better answer for you.

I have found a workaround. Disabling this plugin and enabling Domain Mapping System (https://www.ads-software.com/plugins/domain-mapping-system/) does fix the issue, though it’s not an ideal solution because with Domain Mapping System things like subpage mapping seem to be a premium feature (https://domainmappingsystem.com/#pricing) whereas with your plugin it’s included for free. Also with DMS the page titles don’t appear correctly, it just shows the main site tagline.

With some more investigation it looks like you can use Domain Mapping System and Multiple Domain Mapping on Single Site at the same time. So you can use just DMS for affected domains while still using this plugin for domains that still work. You can even leave the domain mapping for the affected domain in both plugins and it looks like DMS overrides this plugin for that domain.

However, DMS still has the problems I mentioned earlier. DMS also doesn’t work with setting a custom favicon for the specific page by adding the following code to the specific page’s header: https://www.ads-software.com/support/topic/a-way-to-have-custom-site-icon-for-any-pages-including-mapped-pages/

I also wonder if Domain Mapping System restarted their development after many years of no updates in response to the popularity of this plugin.