This is not an answer, it is a description of the problem. I am assured that this will be addressed soon in a proper manner.
The way that
pgfpages (and its extension,
pgfmorepages - which I maintain) works is by saying to LaTeX, "You build the entire page exactly how you want to, then just before you ship it out, I'll take it and squirrel it away for later consideration.". Then at a later point, they say "Right, now I want you to ship out exactly this page.".
From that description, it is hopefully clear that
pgf[more]pages needs to interrupt the shipout routine at the very, very last moment. The page needs to be complete, including headers and footers, and backgrounds, and borders.
From poking around in the
latex.ltx code, I can see that the LaTeX team have been working behind the scenes to streamline the LaTeX core. Part of that involves putting hooks at various points to allow packages to hook in to various routines in a more orderly fashion than has been previously possible. I guess that the purpose of this is to sort out the mess that is conflicting packages who all try to redefine the same command, environment, or routine.
With regard to the shipout routine, therefore, the new LaTeX kernel adds in its own code at the point of shipping out a new page which provides access to all of these hooks. One of which, by the way, is used by
geometry to put the frame around the page (
geometry doesn't know this - thanks to the wizardry of the LaTeX team an awful lot of packages should just work without modification).
So at the moment we have a race condition in that both LaTeX and
pgf[more]pages are trying to insert their code into the shipout routine and both work by replacing
\shipout by their code. The order that should happen is that LaTeX's code works first and then
pgf[more]pages second. Unfortunately, as LaTeX is loaded before
pgf[more]pages, actually what happens is that
pgf[more]pages is executed first and then the LaTeX code. This means that code such as
showframes is only executed when the page is actually shipped out and not when
pgf[more]pages stores the page. This is why the frames are incorrect.
What appears to be needed is a "Right before shipout" hook that allows
pgf[more]pages to swoop in at the last minute. That currently doesn't exist, however from comments from the LaTeX team it would appear that they are aware of the issue and will address it shortly.