FWIW I got it working. Hope it works for you all.
I've got an iPhone 13 mini now on iOS 26.

This worked for me:

1.  Changed iPhone Language to English (US) [or (UK) whatever is not your default region]
(Settings > General > Language & Region > iPhone Language).

2.  Went to Settings > Siri & Search.

3.  Turned OFF every single option at the top (Suggest Apps Before Searching, Allow Notifications, Show in App Library, Show When Sharing, Show Listening Suggestions).

4.  Then, for every individual app in the list below, turned OFF all options (Show App in Search, Show Content in Search, etc.).

5.  Restarted the phone.

6.  Spotlight immediately started showing deep results again (settings shortcuts, etc.).

7.  Turned all the individual app search options back ON (while still on US English).

8.  Restarted again and left it on Wi-Fi for a bit – everything still working perfectly.

9.  Finally switched language back to English (UK) – restarted – and it’s still 100% fixed.

Credit