Hi!
No, the actual numbers don't appear to be important. I simply picked some numbers to make sure the frame was wide enough for two side-by-side windows. I tried 40 instead of 70 and you get the same effect.
I really doubt that the code in `follow-scroll-up' is broken. Follow-mode is designed so that the rearrangement of the other windows (the ones that follow the selected window) occur in the post-command hook (to allow Follow-mode to act upon all Emacs commands, not only it's special function). It appears that something has changed in the display engine, or in the way that post-command-hook is called, that makes this mechanism fail. This could also account for the difference we see when the function is called via a key sequence as compared to via M-x.
This is also the reason why reported this as a bug, rather than digging into the code myself. However, I could try to add log code to Follow mode, to check if I could try to figure out what is going on.
-- Anders