This stack overflow answer helped me:

Expo eas build error: Xcode 16.3 and undefined template 'std::char_traits<unsigned char>' prevent building the app

I didn't realize that when I use EAS to build my app, EAS uses a specific Mac OS and Xcode version as specified in my app's eas.json file. Because I had the production iOS image set to "latest", EAS was using Xcode 16.3 to compile my app.

I was thinking that when I used EAS to build my app, it sent the version of Xcode that I had on my local computer to EAS to build with that.

This documentation was helpful to learn about the image options available when using EAS to build an app:

https://docs.expo.dev/build-reference/infrastructure/