This is a side effect of upgrading CA PPM 15.2 to CA PPM 15.4, when PMO Accelerator is in use.
The default Tabs - and many other aspects - may reset, and you will need to revert them to your preferred format.
CA PPM SaaS - 15.4 - Add-Ins and Integrations
"The 'System: Clarity content' addin is installed by default. This includes updated versions of the processes, queries, portlets, groups, jobs, menu items, and pages listed on the Items tab. Most of these will install by default or have a Status as 'Upgrade Ready'. If they are 'Upgrade Ready', if and when you decide to install them, with the 'Install' button at the bottom of the page. If there is any customizations done to that component in your system, it will be overwritten with the out of the box configuration."
Alternatively, please create a Custom Portlet for your users, which will not be affected by the upgrade.
How do I hide the 'Financial Plan', 'Hierarchy' or 'Processes' tab on the Project object?
"It is recommended not to deactivate tabs on the 'Project Default Page Layout', but instead to make a copy and use the new one for this purpose. To deactivate the individual tabs, use the following steps: . . ."