References to custom fields don't appear to be working

**Describe the bug**

I have a custom field `foo` and tried to reference it in the username as `{S:foo}`, but the Magikeyboard, the autofill entry and the KeePassDX app itself all show and use the literal string `{S:foo}` instead of the actual value of `foo`.

**Keepass Database:**
- Created with: KeePassXC
- Version: 2.x
- Location: Local file synced via Syncthing
- Size: ~250kB
- Contains attachment: Entry no, DB yes

**KeePassDX:**
- Version: 2.8.3
- Build: libre
- Language: German

**Android:**
- Device: Oneplus 3T
- Version: LineageOS 17.1, Android 10

**Additional context**
Add any other context about the problem here.
- Browser for Autofill: Firefox 80.1.2

Reproduce with a KeePassXC database containing a custom field named `foo`, then check the username in KeePassDX, Magikeyboard, and the autofill flow using Firefox 80.1.2. Trace how `{S:foo}` is handled in each path; done means all three show and use the custom field value instead of the literal reference.

Written by the indexing model from the issue text.