CI and all developer environments had zero issues until a single laptop environment stopped being able to build the iOS app. The error thrown was Library not loaded: @rpath/Foo.framework/Foo
Adding this Foo framework manually via Xcode resolved the problem.
HOWEVER, it broke the builds on CI and all other developer environments. Those are now throwing the error
Error (Xcode): Multiple commands produce '/Users/admin/Library/Developer/Xcode/DerivedData/Runner-adwcstkmsmzcjgdzgisfheqwwytp/Build/Intermediates.noindex/ArchiveIntermediates/Runner/InstallationBuildProductsLocation/Applications/Runner.app/Frameworks/Foo.framework'
So we have a seesaw situation where fixing the issue on one environment breaks it on the others.
We have checked the versions of all major pieces of the toolchain (Xcode, iOS, Flutter, Cocoapods) and they are the same across all environments. We have tried reinstalling Xcode and using a fresh repository on the problematic laptop environment.

This answer may not resolve this for everyone, but in my case it did resolve the problem.
Apparently this line was added to my
.zshrcat some point, I think it was recommended by brew during installation of something. In any case, I was getting errors when trying to runarch -x86_64 pod updateuntil I removed this: