Metal API Validation assertion "missing Buffer binding" during draw in New Runtime (6.21.1)

[khaptonstall]

Submission checklist

• I have confirmed the issue is present in the latest version of RiveRuntime
• I have searched the documentation and forums and could not find an answer
• I have searched existing issues and this is not a duplicate

Description

After upgrading from 6.20.5 to 6.21.1, we've started hitting a Metal API Validation assertion when rendering with the New Runtime. It only occurs when "Metal API Validation" is enabled in the scheme's Run → Diagnostics options; with validation disabled the app runs and the animations render fine.

The assertion fires from the Metal command encoder at draw time. Two of the vertex shader's buffer arguments (indices 6 and 7, TC and PB) are reported as unbound. Because it's a hard assertion, the process aborts.

This does not reproduce on 6.20.5, so it appears to be a regression introduced somewhere in the 6.21.x line.

Console output:

-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:], line 6001: error 'Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): missing Buffer binding at index 7 for PB[0].
'
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:], line 6001: error 'Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): missing Buffer binding at index 7 for PB[0].
'
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:6001: failed assertion `Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): -[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:6001: failed assertion `Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): missing Buffer binding at index 7 for PB[0].
'
missing Buffer binding at index 7 for PB[0].
'
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:6001: failed assertion `Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): missing Buffer binding at index 7 for PB[0].
'
CoreSimulator 1051.55 - Device: iPhone 17 Pro (ADDB7915-F08F-4C7E-BAA7-9861662F534E) - Runtime: iOS 26.4 (23E244) - DeviceType: iPhone 17 Pro
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:6001: failed assertion `Draw Errors Validation
Vertex Function(YB): missing Buffer binding at index 6 for TC[0].
Vertex Function(YB): missing Buffer binding at index 7 for PB[0].
'

Previous working version

6.20.5

Rive Apple runtime version

6.21.1

Reproduction steps / code

1. Edit the run scheme → Run → Diagnostics → enable Metal API Validation.
2. Build and run a Debug build on the iOS Simulator.
3. Present a view that renders a .riv file using the New Runtime (Rive Renderer / Metal backend) — e.g. Tests/Assets/data_binding_test.riv from this repo (https://github.com/rive-app/rive-ios/blob/main/Tests/Assets/data_binding_test.riv).
4. On draw, the validateCommonDrawErrors assertion above fires and the app aborts.

With Metal API Validation disabled, the same flow renders correctly.

Upload your reproduction files / stack trace

No response

Source .riv / .rev file

Tests/Assets/data_binding_test.riv (from this repo)

Screenshots / video

No response

Device

iOS Simulator (iPhone 17 Pro)

Apple OS version

iOS 26.4

Additional context

Only reproduces with Metal API Validation enabled; does not reproduce on 6.20.5.