Overflow Hidden + Custom Border Radii does not work on iOS

Description

I've discovered issue with custom border radii (i.e. not all borderXxxRadius being equal), when used with overflow: hidden & solid background. The issue occurs only on iOS.

Steps to reproduce

1. Install repro from: https://github.com/mdjastrzebski/rn-repro-overflow-hidden-custom-border-radius
2. Run on iOS

React Native Version

0.81.4

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

System:
  OS: macOS 15.6.1
  CPU: (11) arm64 Apple M3 Pro
  Memory: 84.83 MB / 36.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.17.1
    path: ~/.nvm/versions/node/v22.17.1/bin/node
  Yarn:
    version: 3.6.4
    path: ~/.nvm/versions/node/v22.17.1/bin/yarn
  npm:
    version: 10.9.2
    path: ~/.nvm/versions/node/v22.17.1/bin/npm
  Watchman:
    version: 2025.07.28.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /Users/mdj/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 24.5
      - iOS 18.5
      - macOS 15.5
      - tvOS 18.5
      - visionOS 2.5
      - watchOS 11.5
  Android SDK:
    API Levels:
      - "35"
      - "36"
    Build Tools:
      - 35.0.0
      - 36.0.0
    System Images:
      - android-35 | Google Play ARM 64 v8a
      - android-36 | Google Play ARM 64 v8a
    Android NDK: Not Found
IDEs:
  Android Studio: 2025.1 AI-251.26094.121.2512.13991807
  Xcode:
    version: 16.4/16F6
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.16
    path: /usr/bin/javac
  Ruby:
    version: 3.4.5
    path: /Users/mdj/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.1.0
    wanted: 19.1.0
  react-native:
    installed: 0.81.4
    wanted: 0.81.4
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: false

Stacktrace or Logs

N/A

MANDATORY Reproducer

https://github.com/mdjastrzebski/rn-repro-overflow-hidden-custom-border-radius

Screenshots and Videos

iOS

Android

まず、https://github.com/mdjastrzebski/rn-repro-overflow-hidden-custom-border-radius の必須の再現環境をインストールして iOS で実行し、その後 Android と結果を比較します。Issue ではリポジトリのファイルもテストも指定されていません。完了条件は、iOS で overflow hidden と単色の背景を使用したカスタムの border radius が正しく描画されることです。