Cached .build always recompiles on CircleCI

Using XcodeGen version 2.6.0 at f63f652988355145d3a56ea6bf7257d42e6fdbb7 on CircleCI 2.0

I am running swift run xcodegen in it's own job on CI and caching the .build output for reuse in my build job. Whenever I restore the cache, and then run swift run xcodegen generate, it will recompile the build dependencies.

Here are the yaml aliases I've defined:

restore_xcodegen_cache: &restore_xcodegen_cache
  restore_cache:
    key: xcodegen-{{ .Environment.CIRCLE_WORKING_DIRECTORY }}-{{ .Branch }}-{{ checksum "../Package.resolved" }}

xcodegen_deps: &xcodegen_deps
  <<: *defaults
  steps:
    - *shallow_checkout
    - *restore_xcodegen_cache
    - run:
        name: Build xcodegen dependencies if needed
        command: if [ ! -d "../.build" ]; then swift run xcodegen; fi
    - save_cache:
        key: xcodegen-{{ .Environment.CIRCLE_WORKING_DIRECTORY }}-{{ .Branch }}-{{ checksum "../Package.resolved" }}
        paths:
          - ../.build/
    - *chat_failure_notification

xcodegen: &xcodegen
  run:
    name: Generate project file with Xcodegen
    command: |
      #!/bin/sh
      swift run xcodegen generate

I'm using the above aliases in my build job:

build: &build
  <<: *defaults
  steps:
    - *shallow_checkout
    - *set_carthage_env_var
    - *restore_cache
    - *restore_xcodegen_cache
    - *xcodegen

My thought would be that running swift run xcodegen generate after restoring the .build directory, would lead to just generating the project but instead the dependencies are recompiled. Does anyone have any ideas/solutions for this? When I perform the same steps locally, the call to swift run xcodegen generate will just generate the project and not do any recompilation. This adds ~35 seconds to my build step - would be nice to avoid that.

Any thoughts/suggestions would be greatly appreciated!

Start with the CircleCI job aliases and the commands swift run xcodegen and swift run xcodegen generate; compare the restored .build directory and Package.resolved between the dependency and build jobs. Reproduce the cache restore on CircleCI and determine why dependencies recompile, then verify that project generation completes without recompiling them.