You could potentially deal with this in 2 ways.
First would be a visual indicator (a small colored bar with an arrow pointing down saying “more entries in this month”, giving a cue that there is more information to be found. That indicator could also be in the category budget box (like a little warning icon and a small text).
“Snap” to a month, where scrolling the mouse would always show the full active month, and scrolling down would basically be going monthly. This would probably almost always result in one month per scroll however, so maybe that’s what you should do, simply (on a visual note, it would be great to actually “scroll” it in a smooth/bouncy way, not just to refresh the page).