pip cache isn't working correctly

I am using the pip cache, as configured in the bitbucket-pipelines.yml file:

caches:
  - pip

The cache is built properly, but when it is downloaded and unpacked, something goes wrong and the http directory of the cache is empty.

I know for sure that on the previous run the pipeline downloaded packages to the http cache and uploaded the cache successfully:

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When it is downloaded it is the correct size:

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But if I look at the pip cache directory, something is missing (the total size is only 5.6M!):

Where did the http cache go?