I'd done a search engine search using two engines for any "public" use of tab-bar-tab-post-open-functions and found only two cases, both of which would be affected positively by the patch. I'm guessing that there aren't a lot of people using the hook or the missing call for the initial tab would have cropped up sooner.

On Wed, Oct 30, 2024 at 3:35 AM Juri Linkov <juri@linkov.net> wrote:
> Plus, the patch treats both initial and secondary tab opening identically
> with no special casing. That's key for users.
>
> On Tue, Oct 29, 2024 at 4:04 PM Ship Mints <shipmints@gmail.com> wrote:
>
>     But the patch works and tabs have been set and the initial tab has been
>     assigned and passed to the hook. Works fine. I'm missing your concern.

The only concern here is that this change might break existing users' configs.
The users already might have a lot of different unimaginable uses of
this hook such as for example to move a new tab to another position
on the tab-bar, etc.  Are you sure this change won't break users' configs?