Custom site id

If you are not using WP-Piwik as a network plugin, you can just disable the site auto estimation and choose the site to use.

Otherwise, you can also add your several domains to your “central” Piwik site:
https://piwik.org/docs/manage-websites/#track-subdomains-multiple-domain-names
After removing all the sites you don’t want to use (and maybe after resetting WP-Piwik), WP-Piwik should use the same ID for all added domains.

Ah, yes, I overlooked the “Select site” option. Sorry and thanks!