It is the responsive formatting of the site. If you go to the site via a cell phone and the site has to reformat for a narrow format, it drops the page #'s that you are used to. If you browse the site using a tablet or PC, the page #'s are still there. You can still click on a thread and browse it that way...
You can also try unlocking your portrait versus landscape settings on your phone, and rotating your phone to view the site in landscape. Depends on the phone resolution, but you might widen the site enough that the page #'s pop back into existence.
I set the guidelines for what resolutions impact the site display... but to be honest it is very difficult given the large number of devices that now access the site. 50% of the traffic is now via cell phone.