React Native iOS - Hermes is enabled in Podfile, but isn't enabled in app

I'm trying to enable Hermes in my React Native 0.65 app for iOS. I've followed the documentation to enable Hermes by making this change to my Podfile:

use_react_native!(
     :path => config[:reactNativePath],
     # to enable hermes on iOS, change `false` to `true` and then install pods
-    :hermes_enabled => false
+    :hermes_enabled => true
   )

Then I cleaned the build folder, ran pod install, and rebuilt my app for a physical iOS device in debug mode. However, running the following code provided by React Native in my app shows that Hermes is actually disabled:

const isHermes = () => !!global.HermesInternal; // <-- returns false

How can I enable Hermes in my app? I'm running React Native 0.65.1 and recently upgraded from 0.64.2. Thank you.