In Classic ERA, post-1.14 the buff runs off when exiting BRS.
Not intended, as far as I know. Please write this down on you to-do list!
But the forums are not the place for reporting bugs, therefore there is no “to do” list. Nor will the forum moderators forward bug reports to the appropriate department, because if they started doing that, that’s all they’d do all day long, instead of the work they are actually paid to do.
Instead you need to make a bug report from in-game, via the bug reporting tool.