Photo sketch geometry changes all strokes to the last selected colour when using tap-to-draw method

Description
When drawing photo sketches using the tap-to-draw method (tapping on the screen to place points, with lines created automatically between them), each time the user selects a new colour, all previously drawn strokes change to the newly selected colour. This issue does not occur when drawing freehand directly with a finger — in that case the colour is correctly preserved per stroke. Only the tap-to-draw method is affected.

Environment
[x] Staging

Application (+ app version, build, operating system)
Mergin Maps Mobile App

• iOS version: 2025.6.0 — iOS 26.0
• Android version: 2025.6.0 — Samsung Galaxy A53 5G, Android 15
• Windows application

Steps to reproduce

1. Open a project with photo sketching enabled.
2. Open a feature form and attach a photo from the camera or gallery.
3. Tap the photo sketches icon to open the sketching toolbar.
4. Select a colour (e.g. blue).
5. Tap on one location on the photo, then tap on another location approximately 2 cm away — a line is automatically created between the two points in blue.
6. Select a different colour (e.g. green).
7. Tap on a new location to create another line segment.
8. Observe the colour of the previously drawn blue line.

Actual results
After selecting green and drawing a new tap-to-draw stroke, the previously drawn blue stroke changes colour to green. The entire sketch geometry adopts the most recently selected colour. Previously drawn strokes do not retain their original colour when using the tap-to-draw method.

Expected results
Each stroke should retain the colour selected at the time it was drawn, regardless of the drawing method used. Changing the colour for a new stroke should not affect any previously drawn strokes.

Note
When drawing freehand with a finger, the colour is correctly preserved per stroke — the issue is specific to the tap-to-draw interaction method.

Screenshots or logs

https://github.com/user-attachments/assets/1f414d1e-e983-433c-95a0-3ad145ed46ef

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

Reproduce the issue through the photo sketches flow using the tap-to-draw method, comparing it with freehand drawing and different colour selections. Trace how tap-created line segments store their colour versus freehand strokes. Done means each stroke retains the colour selected when it was created on iOS, Android, and Windows.