Unable to reserve cache (actions/cache/save@v5)

When using actions/cache/save@v5 to rewrite the existing cache, the action ends up with the following error:

Failed to save: Unable to reserve cache with key XXX, another job may be creating this cache.
Warning: Cache save failed.

I've seen similar issues like #342, but when you look at the documentation of https://github.com/actions/cache/tree/main/save#case-1---where-a-user-would-want-to-reuse-the-key-as-it-is, the example clearly shows that the cache key CAN be reused and overwritten.

Could someone explain if it's something I am doing wrong or if it's something that is being actively overlooked in this action?

(I am almost 100% sure that there is no race condition on my side as I only use 1 workflow with a single job and I am explicitly using actions/cache/restore and actions/cache/save.)

Thanks!

Start with the actions/cache/save@v5 and actions/cache/restore usage described in the issue, then compare it with the linked save documentation example for reusing a key. Reproduce the failure in a single-job workflow and determine whether key reuse is supported; done means the behavior is explained and the documentation or action behavior has a clear resolution.