Crash on old iOS devices (IL2CPP-related symbol not found)

I'm building an empty, new unity project into an xcode project with il2cpp.
It works just fine for all x86_64 simulators, but in order to try it on something older than an iPhone 5s, for example a simulated iPhone 5, I need to add another architecture in xcode (i386 or armv7) which lets me select iPhone 5 as a simulator. I'm guessing these older phones are 32-bit systems or something?
Whenever I run a simulator using another architecture, it crashes as soon as the app opens, with the error message dyld: Symbol not found: __ZN14Il2CppIUnknown3IIDE
It works if I build using Mono, but that's not really an option because it won't be allowed on the app store then.

What's going on?

I don't have another Mac to build with, so I'm not sure if it's a device-specific build issue or not.
I've tried running it on an actual iPhone 5, but it crashes as soon as it opens there as well.
I've with both Device SDK and Simulator SDK as Target SDK, and turning of "Strip Engine Code".
I've tried it on both Unity 2017.3.1f1 and Unity 2018.1.0f2. I'm using Xcode 10.1
Any help would be greatly appreciated!

 

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