I just noticed this today as well. Installed a new Tableau 2018.2.2 server on CentOS 7.5. Uploaded the proper SSL certs and the TSM :8850 url says it's serving a self-signed certificate from the TableauServerManagerCA. I can't get it to properly serve the officially signed SSL running on :443.
Far as I'm aware this is expected behaviour in the current release:
Note: Tableau Server creates and configures a self-signed certificate during the installation process. This certificate is used to encrypt traffic to the TSM Web UI. Because it's a self-signed certificate, your browser will not trust it by default. Therefore, your browser will display a warning about the trustworthiness of the certificate before allowing you to connect.
I'm hoping they will give us a way to fix it in this release....that's poor design in my opinion.