The blue back button in the vertical timeline (or on mobile, it pops up above the timeline docked at the bottom) takes you to your lowest read position in the topic.
(I am testing in https://bbs.boingboing.net/t/post-your-pet-or-animal-pics/72032 which I do have read position on, it is post #1983)
However if your lowest read position is at the absolute bottom of the topic, the button will not appear because you can already reach the bottom by pressing the end key or clicking/tapping the bottom of the timeline.
Do note that there has to be room for it to physically appear, so you must scroll up “enough” for it to be a meaningful jump, which depends somewhat on post count.
I can’t repro any problem with “clicking the button several times”. In the above topic every time I click the blue back button on the timeline, I am taken to my last read position.