Multisite multiple domain redirect issue with "com" at beginning of URL?

I’m having a weird issue with my multisite installation and the multiple domain mapping plugin. I’ve got all my domains added and all the content is pulling into the correct site and everything is resolving correctly when I navigate directly to the sites’ URLs individually. However…

All the sites that aren’t my main site seem to be having a weird domain issue when navigating to them from the “My Sites” menu at the top. For some reason, it’s mixing up the order of the URL and adding “com” at the beginning of the URL like so:

https://www.mymainsite.com/sitedirectory

is linking to

https://com/sitedirectory/mymainsite

(which obviously doesn’t work)

Like I said, the sites all work when I navigate to them directly and I can manage them ok and update content pretty well, but any plugins that I have installed try to link to that funky URL for their stylesheets and script files, causing issues.

My htaccess file is pretty standard, just what WordPress says to put in:

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

# END WordPress

And my wp-config file is the same way (just what WordPress ordered):

define('WP_ALLOW_MULTISITE', true);

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'www.mysite.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

define( 'SUNRISE', 'on' );

Anybody have any idea what’s going on here? Any help provided would be much appreciated.