I am 100% positive, take a look:

$ git version 
git version 2.21.0

$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

    a/a2
    b

nothing added to commit but untracked files present (use "git add" to track)

$ git add .

$ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

    new file:   a/a2
    new file:   b

Milica Maksimović ^

And it works even if the file in the subdirectory is not new but modified:

$ git status
On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

    modified:   a/a2

no changes added to commit (use "git add" and/or "git commit -a")

$ git add .

$ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

    modified:   a/a2

Yup, you're right. I guess this was the case in the past. I'll edit the original comment! Thanks!