Chromium treats a small area at the top of inactive tabs as a space for dragging the window. If you click that area, an inactive tab won’t become active. It turns out that for Brave’s new horizontal tab design, this area was too big and really noticeable. We have a fix on the way.
Thanks for reporting this, and thanks for using Beta! 
This was super helpful feedback.
Agreed. Hope they fix it.
On Chromium you can select the tab when the window is maximized even if you drag the mouse all the way to the top of your screen. Check the following video (flag #chrome-refresh-2023 is enabled):
On Brave you cannot do that. So the issue is not just the amount of space.
Does chrome exhibit this behavior without the refresh flag enabled?
Yes it does. I even tested on Chrome Canary and the results were the same.
There are a few lines of horizontal pixels at the top of the tabs that don’t change to the tab, instead registering as clicking on the frame of the browser window. This is only happening on Beta for Linux. Windows version will switch tabs.
That being said, the new tabs are awesome. It’s about time. The old ones are way too small on 1440p monitors. Now I wish they would fix the small font in the address bar on Linux.