Unable to find a specification for a local (private) pod

Question

I created two local pods: MainPods & ChildPod like this:

1. Create Framework (in Xcode)
2. pod init & pod install for MainPod & ChildPod
3. pod spec create MainPod & pod spec create MainPod (in their folders)
4. modify it

Then I inited pod in root folder of application: platform :ios, '13.0'

target 'PrivatePodBugDemo' do
    use_frameworks!
    # pod 'ChildPod', :path => 'DevPods/ChildPod'
    pod 'MainPod', :path => 'DevPods/MainPod'
end

MainPod.podspec ChildPod.podspec

What did you expect to happen?

I expected to all run good when I include only MainPod which already includes ChildPod.

What happened instead?

This error happened [!] Unable to find a specification for ChildPod depended upon by MainPod

It's disappears when I uncomment pod 'ChildPod', :path => 'DevPods/ChildPod' in app's Podfile. But I need to not include ChildPod in app's Podfile.

Project that demonstrates the issue

https://github.com/ocbnishi/PrivatePodBugDemo

https://github.com/CocoaPods/CocoaPods/issues/9803