Added key.toLowerCase(): when I press shift-Z the key is "Z" not "z". Confirmed at https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key with Chrome, Safari and Firefox.

Ugh, it appears that Chrome doesn’t implement this correctly on macOS. I’ll revert back to the legacy keyCode for now.

(the behavior was the same on the 3 browsers, it was not only Chrome)

For me, Safari, Chrome, and Firefox all return lowercase “z” on macOS when shiftKey is down, which deviates from the documented “taking into consideration the state of modifier keys”. I can’t reproduce the behavior you describe, but it is consistent with the linked documentation.