Expired Domain Lost Backlink

I purchased an expired domain, after a few days, it's backlink status is lost when i checked on ahref.

The explaination on the lost backlink is

" The linking page redirected to a different URL during the most recent crawl. If the redirect destination page also has a link to the target URL, this links will appear as live backlinks. "

Is it because the backlink has it's link to https://www.mydomainname.com/ ?
My domain now is only displaying https://mydomainname.com/ without www

Yes, the loss of the backlink may be because of www version and non www version

When a backlink points to https://www.mydomainname.com/, but yours displaying as https://mydomainname.com/ - it can create a mismatch.
Some tools like Ahrefs may take it as different unless you have proper redirects in place.

To avoid losing backlinks and ensure consistency, you should update 301 redirect and update backlinks if possible. Contact the webmasters the site and ask them to update your old url to new url

Implement 301 redirects from https://www.mydomainname.com/ to https://mydomainname.com/. This will ensure that any traffic or link equity from the www version is properly redirected to the non-www version of your site.If possible, try to get the backlinks updated to point directly to the https://mydomainname.com/ version of your site. This can sometimes be done by reaching out to the sites that have linked to the old www version.

Yes, the issue is likely due to the difference between the "www" and non-"www" versions of your domain. When a backlink points to the "www" version (e.g., https://www.mydomainname.com/) but your site is set up to use the non-"www" version (e.g., https://mydomainname.com/), it can cause the backlink to be lost during a crawl.

To resolve this, set up a 301 redirect from the "www" version to the non-"www" version. This will ensure that all backlinks, regardless of the version they point to, are correctly attributed to your site.