Multi-select filter returns incorrect number of features when multiple values are selected

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**Description**

When a multi-select filter is configured on an array-type text field (e.g. "Favorite cities") and the user selects multiple values (e.g. Vienna and Prague), the filtered feature list shows fewer results than expected. Features containing either of the selected values should all be returned.

**Environment**
[x] Staging [ ] PROD

**Application (+ app version, build, operating system)**

- Mergin Maps mobile 2026.3.0, Android 16 (Pixel 9)
- Mergin Maps mobile 2026.3.0, iOS 26.0

**Steps to reproduce**

1. Clone and open project `tester-ws-1/test_feature_filtering` in the mobile app
2. Open Filters → select filter `Multi select-photo_survey-Favorite cities`
3. Select values: **Vienna** and **Prague**
4. Tap **Apply filters**
5. Open the `photo_survey` features list

**Actual results**

2 features are shown.

**Expected results**

3 features are shown — all features whose "Favorite cities" field contains Vienna or Prague:
- Monica Geller — {Bratislava, Vienna}
- Chandler Bing — {Bratislava, Prague}
- Janice Hosenb... — {Budapest, Helsinki, Prague}

**Screenshots or logs**

QGIS attribute table confirming expected 3 matching features: *(see attached)*

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

Reproduce the issue in the Mergin Maps mobile app using project tester-ws-1/test_feature_filtering, then trace the Filters flow for the multi-select-photo_survey-Favorite cities filter into the photo_survey features list. Compare the filtering behavior for Vienna and Prague with the QGIS attribute table. Done means all three features containing either selected value are shown on Android and iOS.

Written by the indexing model from the issue text.