As you can see in the screenshot, some tabs use white text for tab title while others are using black/dark text. Switching to those tabs or hiding and showing the sidebar does not fix the issue.
It might be related to macOS switching between light mode and dark mode automatically every night and morning. This browser window have been open for a while and has been through a number of those switches.
I expect all tab titles to use the same color and it be a color that is easy to read on the current background.
Version 0.99.129-beta (WebKit 619.1.11.111.2)
Sequoia (15)