Let's Encrypt sends certificate renewal notices to all user accounts that have a certificate on the same host.
In Let's Encrypt version 1.x all certificates were issued using one Let's Encrypt account.
In Let's Encrypt version 2.x certificates are created in separate Let's Encrypt accounts per Plesk subscription.
This is Plesk bug with ID #PPPM-4358, which was fixed.
Update the extension at Plesk > Extensions.