Hi,
I noted from the Current Channel release notes that this feature has just been released to non-Insider users, so I installed MS365 updates (some were downloaded and installed) and gave it a try... but nothing happened. Clicking on a function parameter in the tooltip to select it, and hovering my mouse cursor over the selected parameter/formula part does not produce a tooltip with a preview of the evaluation of that part of the formula.
I'm on Microsoft 365 Apps for business, Version 2303, Build 16227.20212 Click-to-Run, Current Channel.