Setting the custom budget period to "New budget every 1 month" with the "Last day of the month" toggle enabled causes transactions dated on the last day of the month to be excluded from the totals displayed in the Spending Breakdown on the Overview page when looking at "This month".

For example, if you have two income transactions of $100 categorized under Income (one dated June 15 and another dated June 30), the 'Total Earned' value for the Income category in the Spending Breakdown will show only $100 instead of the expected $200 for "This month" (June 1 to June 30).