Expression-based field not recalculated when value relation field changes on mobile (Apply default value on update)

Description

A non-editable form field (Placeholder1, alias Country-Continent-Capital) uses the default value expression "Placeholder2" || ' - ' || "EU Member" || ' - ' || "Capital" with Apply default value on update enabled. In the mobile app two issues are observed:

1. On new feature collection: The Country-Continent-Capital field is not always populated after selecting a country — it sometimes remains empty.
2. On country change: When the Placeholder2 (Country) field — a value relation from a non-geometry layer — is changed, only the country portion of Country-Continent-Capital updates; continent (EU Member) or capital (Capital) retain values from the previously selected country.

Environment
[x] Staging [ ] PROD

Application (+ app version, build, operating system)

• Mergin Maps iOS 2026.1.3 (26.3.884911), iOS 26.0
• Test project: tester-ws-1/test_default_values

Steps to reproduce

Symptom 1 — field not populated on new feature:

1. Open project tester-ws-1/test_default_values in the mobile app
2. Add a new point feature
3. Select a value for the Country (Placeholder2) field
4. Observe Country-Continent-Capital (Placeholder1) — it is sometimes empty

Symptom 2 — stale values after country change:

1. Open project tester-ws-1/test_default_values in the mobile app
2. Add a new point feature and select a country (e.g. Armenia) — Country-Continent-Capital populates as Armenia - Asia - Yerevan
3. Change the Country field to a different value (e.g. a European country)
4. Observe Country-Continent-Capital — only the country portion updates; continent and/or capital remain from Armenia

Actual results

• Country-Continent-Capital is intermittently empty on new feature collection
• After changing the value relation field, the concatenated field shows stale continent and capital values

Expected results

With Apply default value on update enabled, the Country-Continent-Capital field should fully recalculate whenever any of its source fields change. This works correctly in QGIS desktop.

Screenshots or logs

https://github.com/user-attachments/assets/5dcd045d-f656-4e4f-9654-2d73179b2aac

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