I am using JIRA Cloud (JIRA v7.1.0-OD-02-030). Oddly, that is in the "Unreleased Versions" of the Affects Version/s list.

While trying to resolve an issue, I found that the toggle from Edit mode to Preview did not function at all, even though I tried clicking on it multiple times.

Then I tried entering a normal comment in a JIRA issue. The toggle worked to go from Edit mode into Preview mode. But then it became somewhat stuck in Preview mode. Clicking the Edit mode icon did not switch back. However, after clicking the Edit mode icon several times it finally did revert back to Edit mode. It seems probabilistic on whether it works or not. (Also at times it seems as if it tries to go into Edit mode and immediately reverts back to Preview mode. At other times there is no visible response at all to the click.)

This behavior is repeatable.

I also observe that when I mouse over the help icon, I get a mouse-over tool tip "Get local help about wiki markup help" and the browser (Chrome) identifies that there is an associated link. However, when I mouse over the Preview mode icon next to it, there is no tool tip and no recognition by the browser of any link. I do believe that in the past there used to at least be a tool tip message to indicate the distinction of going to Preview mode vs. going to Edit mode. That should be restored as it was helpful to understand the function of the icon/button.