[Bug]: Alternative route line partially missing when overlapping with main route

Mapbox Navigation SDK version

3.10.2

Steps to reproduce

Platform: iOS
SDK version: 3.10.2
Device: iPhone 14 Pro Max(iOS 18.6 / iOS 26)
Map style: mapbox://styles/mapbox/dark-v11

When displaying a long-distance route (hundreds of kilometers) that includes alternative routes, parts of the alternative route line are not rendered on the map.

The issue only occurs for alternative routes, not for the main route.

[REPRODUCE]

1. Request a long-distance route (e.g. 600+ km) with alternatives enabled.

2. Let the SDK render both the main and alternative routes automatically after DirectionsResponse.4.

3. Observe the map view:- Some sections of the alternative route are missing.

   • Switching the same route to become the main route makes the missing parts appear correctly.
   • The missing segment typically corresponds to the portion that overlaps with the main route.

4. Adjust the zoom level:

   • When zooming in, the missing segments gradually appear.
   • When zooming out, the same segments disappear again.

Expected behavior

All routes (main and alternatives) should be rendered completely and consistently, including the overlapping sections, regardless of zoom level or route role.

Actual behavior

Only alternative routes have missing segments.

Missing sections correspond exactly to the overlapping area with the main route.

Switching that alternative to become the main route causes it to render correctly.

The visibility of missing sections changes dynamically with zoom level — visible when zoomed in, missing when zoomed out.

Is this a one-time issue or a repeatable issue?

repeatable

Reproduce the issue on iOS 18.6 or iOS 26 with Mapbox Navigation SDK 3.10.2, a 600+ km route, alternatives enabled, and the dark-v11 style. Start by tracing the route rendering path for alternative routes and their overlapping segments. Done means main and alternative routes render completely and consistently at every zoom level.