Multi site domain link issue

I have two multi site installs and I’m having a domain mapping issue.
Both installs are sub domain installs and use the domain mapping plugin. I’m having problems when linking back to the main domain the network was set up on domain. All the mapped sites use the https://www.domain.com as the primary domain, the https://www.domain.com version of the domain is also set up to point to the WP account.

When linking to the main site domain, if I have just the plain domain.com in the link it does no go to the right page. I get a page holder. If I use the https://www.domain.com in the link it works fine. If I just type the doamin.com into my browser it works judt fine.

I have set up an example page at https://vashon101.com with links to the sites in question so you can see whats going on.

Any ideas whats going on?
Thanks, Steven