`fid` field shown as empty instead of "* Mixed Values *" when editing multiple features

Description

When editing multiple selected features, fields with differing values correctly display * Mixed Values * as a placeholder. However, the fid field (when visible in the form) is shown as empty rather than displaying * Mixed Values *, which is inconsistent with the behaviour of all other fields.

Environment
[ ] Staging [x] PROD

Application (+ app version, build, operating system)

• Mergin Maps mobile 2026.2.0, Android
• Mergin Maps mobile 2026.2.0, iOS

Steps to reproduce

1. Open a project where the fid field is visible in the feature form (or create a new basic project with fid visible)
2. Select multiple features on the map
3. Tap Edit to open form for selected features**
4. Observe the form fields

Actual results

The fid field is displayed as empty. All other fields with differing values correctly show * Mixed Values *, and unsupported fields (e.g. photo) show Not available while editing multiple features.

Expected results

The fid field should display * Mixed Values * consistently with all other fields that contain differing values across the selected features.

Screenshots or logs

Conclusion
Created by Claude and supervised by a human test analyst.

The issue does not identify a source file, test, or entry point. Reproduce the problem by editing multiple selected features with a visible fid field, then locate the form handling for mixed values and verify that fid displays "* Mixed Values *" consistently with other differing fields.