Ok, I figured this out after @MarkSetchell gave me a clue to follow.

What was happening was that, at some point in the past, I moved from a different installation method to npm for the Bitwarden CLI tool.

However, I didn't clean things up properly, and I had two version of the tool coexisting in my system: an old one at /usr/local/bin, and the latest one at /Users/myuser/.npm-global/bin/bw.

In my .zshrc file, my Bitwarden script was running BEFORE the npm path was added to my path variable. Meaning, the script was trying to use the (now very outdated) BW CLI client. And it was probably failing because of an incompatibility with the latest BW features. But because I was not getting a "not found" error, it wasn't easy to notice that I was actually running an older version.

All I had to do was make sure that the npm path was added to the shell before the script is executed, and of course I also cleaned up the old installation to prevent any future confusion.

Thanks everyone for your answers!