Open two tabs only: 1) theverge.com; 2) forums.macrumors.com
Switch between the tabs - which one is active the light or the dark?
I see what you're saying - at last I get it! Well, at least what one person is complaining about.
Assuming you have the MacOS light mode on....
If you turn off "Show color in tab bar" it's consistently the dark one that's the active one. But, if you turn on that setting then The Verge is always the lighter tab, whether or not it's active. It's easy to argue that this is a problem.
What Apple is doing, which is completely consistent with my vision how I process a UI, is making the active tab have a higher contrast against what's behind the tab. If the background is changing from light to dark, the approach to tab shading has to change to maintain that contrast.
If you turn off "Show color in tab bar", do you see then that the active tab is always the darker one? Do you see that the active tab is presented in higher contrast against the background?
A flaw seems to be that Apple is giving us the ability to show varying colors in the tab bar; that is introducing such variability in the tab shading so as to confuse people. That variability in the tab shading has to be there to cater for variability in the background color behind the tabs - assuming their goal is to highlight the active tab using contrast against what's behind the tab.
As you're bouncing around different tabs, the relative approach to shading really should not change, or else you can get lost. So, providing the feature that allows the background behind all of the tabs to change is a mistake. Some people would be annoyed to lose that feature, but it would likely stop a lot of complaining from other people who are accidentally using that feature.
By "contrast", I'm definitely not talking about the contrast between the tab lettering and the background inside the tab itself. My eyes aren't good enough to notice that one (the benefits of poor eyesight). They really should increase the weight of the font when the shading of the tab is darker. As far as my eyes can tell, they don't alter the font weight at all (except when the Safari window goes into the background).