What you are seeing is the new Smart Page feature, which makes PgUp and PgDown look for intelligent landings so that it is easier to orient yourself in the code after paging than it is when using the normal paging by full screens. You can learn more about it by looking at the new features readme.
You can turn it off by going to Tools > Options... > Editing > Cursor Movement > Cursor page up/down > Full Page.
I would like to see the test case (actual file) where it moved only one line. What line did you start on and how many lines were visible in the editor window at the time? Smart Page moving only one line should not happen, unless the editor window is very small. But, it is always possible that you found a corner case.