I guess it’s to show which tab is active. A good way to make it seamless would be to set the background color of the tab in the selected state to blue.
Look at this example: I have 2 tabs, one with the same visual as yours, another with the same color as the outline -
This is an accessibility feature called a “Focus Indicator” and is automatically enabled on all Studio sites. It should only appear if a site visitor uses the Tab key to switch between elements.
That said, the team is looking into a few reports if it not following this case. But shouldn’t interfere with the use of the site