I noticed that for the TEXTJOIN function, a part of the layout of the online help which is shown in Excel is different from the layout shown in the browser. That will lead to that one example provided will not show the same result when using the integrated help view or the help article from the browser.
Please open Excel, open the help task pane and search for TEXTJOIN. Once you have found the article, scroll down to Example 2. The third data row is shown empty. One suggestion would be here to increase the height (e.g. same height as the other rows) of this row for better visualizing, that's an empty row. Please copy the data, as suggested in the article on top, and paste it in Excel. Then, copy the formula, and try both variants (true/false) for the argument ignore_empty. All is fine and it looks like the following screenshot.
Now, please open the online help article in the browser. The browser does not show the empty row at all (and it's not in the html code too). Therefore, if you do the same as above (copy & paste to Excel) from the article in the browser, it won't show the correct result for ignore_empty = false, as the empty row is missing.
I noticed the same layout difference for the CONCAT function. It does not make a difference for the formula results, but in case that the empty row is intentional, then it does not display in the browser.
The layout differences also apply to other languages, such as e.g. German. I tried Chrome, Firefox and Opera as browsers.