The problem:
When tab-line mode is enabled and configured to group buffers, a group
tab is added as the first tab. This is not a regular tab that displays a
buffer, but rather a tab to indicate the group. The face used for this
group-tab is the same of an inactive tab, which makes harder to
differentiate betweem a group tab and a regular tab that can be selected
to display a buffer.

How to reproduce:
1. emacs -Q
2. Configure tab groups by project:
(setq-default tab-line-tabs-buffer-group-function
              (lambda (b) (with-current-buffer b (vc-root-dir)))
              tab-line-tabs-function 'tab-line-tabs-buffer-groups)
3. Enable tab-line mode:
(global-tab-line-mode 1)
4. Open file from a project handled by some vc backend (e.g.: git)
5. The tab group will be displayed as the first tab with a common face