Hi again,
I wanted to provide a quick update: I've figured out how to fix the problem.
Here's what I did: spack clean -a spack providers
I believe what happened was that my Spack installation was using a build configuration cached in "/usr/share/spack/root/opt/.user/cache/cache/providers/builtin-index.json". As a result, any changes I made to my packages.yaml file weren't being applied. The first command cleans this cache along with temporary build files and downloads. The second command then rebuilds the builtin-index.json using my current version of packages.yaml.
I'm still unsure why my upgrade didn't include the zlib-api in packages.yaml, but that's not crucial for now.
Best regards, Tiago Almeida