Cannot dynamically assign new property to $options object in vue/compat

[yuwu9145]

Vue version

3.4.21

Link to minimal reproduction

https://github.com/yuwu9145/vue-compat-options-demo

Steps to reproduce

1. Run the project using vue compat from reproduction link
2. Open console and compare the console log result with the vue build one

What is expected?

Results should be the same

What is actually happening?

Cannot assign a new property to $options object when using vue/compat, but it's available in vue build

VS

System Info

System:
    OS: macOS 14.2.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 158.03 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.19.0 - ~/.nvm/versions/node/v18.19.0/bin/node
    Yarn: 1.22.21 - ~/.nvm/versions/node/v18.19.0/bin/yarn
    npm: 10.2.3 - ~/.nvm/versions/node/v18.19.0/bin/npm
  Browsers:
    Chrome: 123.0.6312.87
    Safari: 17.2.1
  npmPackages:
    vue: ^3.4.21 => 3.4.21

Any additional comments?

No response