As brought up on TeX.StackExchange:
loading the pgf causes the following to be added at the end of the document (in pgfutil-latex.def):
\AtBeginDocument{\AtEndDocument{\par\vfil\penalty-10000\relax\the\pgfutil@everybye}}
As shown in the linked discussions, this can cause extra space to end up at the end of the document, which is a problem when a flush-bottom design is desired.
Fixed https://sourceforge.net/p/pgf/git/ci/fbfabbd01cecb4e8b6b1e43d1f3ad685c7e60778/