A-Z index doesn't show all charactes with content in it

We create a contact structure and have 400 Person in it. We show it in the overview the table Listing and the A-Z Index on top.
But in the index are not all characters show as link (mained they have content), but there are still users with these Name exists?
What was going wrong there?

See Screenshot

Hello

the alpha-index is used only on title of the items

So the title of the items will need to start with the specific letter

It seems that the column you are trying to use is not the title of the item ??

No, that was the Title of the Item ("Oberstuke, Sabine" | "Oberweis, Rolf").
But it was not used in A-Z overview.
Crazy was, if you use the Textsearch and search for "Ober", both items was found and then the a-z index show the activated Character "O"? See Screen.

Hello

i am not sure why this is happening
we do have some code to (optinally) allow listing different items before and after form filtering

e.g. (new feature) you can list only featured items (and e.g. limit listing to 5 items), and after form submit list all items,
but i don't think that the above is related

please send me a PM with superadmin login and the frontend page URL to review this, as it looks like a bug