diff --git a/packages/react-native-mmkv/src/isTest.ts b/packages/react-native-mmkv/src/isTest.ts
index 93429611..f702c70b 100644
--- a/packages/react-native-mmkv/src/isTest.ts
+++ b/packages/react-native-mmkv/src/isTest.ts
@@ -3,7 +3,19 @@ export function isTest(): boolean {
     // In a WebBrowser/Electron the `process` variable does not exist
     return false
   }
-  return (
-    process.env.JEST_WORKER_ID != null || process.env.VITEST_WORKER_ID != null
-  )
+  // Read through a variable rather than as `process.env.JEST_WORKER_ID`.
+  //
+  // Metro parallelises transforms with jest-worker, which sets JEST_WORKER_ID
+  // in each worker process, and bundlers such as babel-preset-expo inline
+  // static `process.env.X` member expressions at transform time -- including
+  // inside node_modules. The transform worker's id therefore gets baked into
+  // the app bundle as a literal, isTest() returns true in a normal debug or
+  // release build, and createMMKV() silently returns the in-memory mock.
+  // Storage then appears to work in-session but is never written to disk.
+  //
+  // Only static member expressions are inlined, so indexing a variable sees
+  // the real runtime environment. Jest and Vitest still set these at runtime
+  // in their workers, so genuine test runs are unaffected.
+  const env: Record<string, string | undefined> = global.process.env || {}
+  return Boolean(env['JEST_WORKER_ID'] || env['VITEST_WORKER_ID'])
 }