If I navigate directly to a JIRA rapid board, e.g. by following a link from an email, using a shortcut in my browser faviourites, or simply typing the URL directly, then I get redirected to my JIRA home page (e.g. issues list or dashboard) if this is the first time that browser instance has visited JIRA .
Once the browser instance has "seen" JIRA the first time, then directly accessing a Rapid Board via a link, shortcut, or URL works as expected. If you close all instances of that browser then the problem recurs the next time you try and go directly to a Rapid Board.
I have "Keep me logged in" selected and I'm not asked for any credentials - it's just routing straight to my configured JIRA home page, rather than to the URL I want.
This is happening in Google Chrome and Microsoft Edge, so I think it's an issue with JIRA rather than weird browser behaviour.