Cocoapod subspec dependency not adding header/library search paths - ios

Question

I'm writing a Cocoapods podspec for a Swift library (https://github.com/promotedai/ios-metrics-sdk). One of our clients asked us to add Firebase Analytics tracking to the internals of this library. As a result, in the podspec, I broke the Firebase dependency into a subspec.
Pod::Spec.new do |s|
s.name = 'PromotedAIMetricsSDK'
...
s.subspec 'FirebaseAnalytics' do |a|
a.source_files = ['Sources/PromotedFirebaseAnalytics/**/*.{h,m,swift}']
a.dependency 'Firebase/Analytics', '~> 7.11.0'
a.dependency 'PromotedAIMetricsSDK/Core'
end
end
The problem I ran into was that the above subspec neither builds nor passes pod lib lint. The build complained about a missing Firebase framework, and the lint step complained about the same thing. I had to add the following xcconfig to the subspec to get things working.
analytics_xcconfig = {
'USER_HEADER_SEARCH_PATHS' => '"${PODS_ROOT}/Firebase/CoreOnly/Sources"',
'FRAMEWORK_SEARCH_PATHS' => '"${PODS_XCFRAMEWORKS_BUILD_DIR}/FirebaseAnalytics" "${PODS_XCFRAMEWORKS_BUILD_DIR}/GoogleUtilities" "${PODS_XCFRAMEWORKS_BUILD_DIR}/nanopb"',
'OTHER_LDFLAGS' => '-framework "FirebaseAnalytics" -framework "GoogleUtilities" -framework "nanopb"'
}
s.subspec 'FirebaseAnalytics' do |a|
a.source_files = ['Sources/PromotedFirebaseAnalytics/**/*.{h,m,swift}']
a.dependency 'Firebase/Analytics', '~> 7.11.0'
a.dependency 'PromotedAIMetricsSDK/Core'
a.pod_target_xcconfig = analytics_xcconfig
end
While the above configuration allows the subspec to build and pass lint validation, it defeats the purpose of having a package manager take care of these dependencies. I have other subspecs that pull in dependencies that don't encounter the same problem. Also, pulling in Firebase/Analytics directly in the app's Podfile works fine.
Am I missing something? Is there a reason why Firebase/Analytics requires this xcconfig hack when building from a subspec of a podspec? Can I get rid of this hack somehow?
For a demonstration of this problem, pull the above repo and remove the a.pod_target_xcconfig = ... lines from the FirebaseAnalytics subspec, then run pod lib lint. Commit 67cb6173ee0f43b33cfb726b6337cc14463d220b in the repo demonstrates a similar problem where the linker can't find the FirebaseAnalytics framework (run pod lib lint on that commit).
I'm using Cocoapods 1.10.1, Xcode 12.5, and macOS 11.3.
Thanks,