When upgrading heroku to the latest stack cache slug is not being used

So I am trying to upgrade Heroku to the latest stack (from Heroku-18 to Heroku-20).

The upgrade seems fine except for one thing.

We are using Review Apps with the Heroku-CI and tests take about 11 min to run. When running the test suite with the new stack it takes like 19 minutes because the test setup downloads stuff into the slug. Instead of using the cache

Example:

In the test setup, a bunch of things is being installed like Postgresql, Redis.

When running the test suite in Heroku CI with the new stack, Redis is being downloaded and not cached. When running the test suite with the previous one (Heroku-18) Redis is being cached.

Part of test setup with the latest stack (Heroku-20)

-----> Redis app detected       Using redis version: 5.0.6-----> Downloading and installing redis into slug  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current                                 Dload  Upload   Total   Spent    Left  Speedredis-5.0.6/

Part of the test setup when the stack isn't upgraded (Heroku-18):

-----> Redis app detected       Using redis version: 5.0.6-----> Fetching redis from cache into slug-----> Redis done-----> Apt app detected-----> Reusing cache-----> Updating apt caches

Tests with the new stack has been run, but this come from a Pull Request that hasn't been merged yet.

Is Redis not being cached into the slug (in the new stack) because this PR hasn't been merged to the main App?

This happens with other downloads like binaries, Redis is an example