Hi,
The different previews is weird. Thanks for bringing it to our attention. I’ll open a bug on it and in the meantime we’ll document it.
Re Preview vs. Publish: Most of the time functionality should be the same, but not always. Certain APIs only work in the Published site, for example some Members APIs. Also some backend events. Any time that is the case we mention it in the reference docs. We also mention it here: https://support.wix.com/en/article/corvid-testing-and-debugging-your-code#testing-in-preview-mode-1
-Jeff