Please take time to give Intuit feedback on this so they will fix it.
When you use the Home Currency feature, it posts the entry to the day you are recording it on even though you enter the correct date, such as December 31, 2016, and the correct exchange rate comes up. When you save it, it posts to the day you are physically doing it.
Find the entry you posted in your P&L and you will see it says today’s date. Change it back to December 31, 2016, and save it. QBO will now keep it. After changing the date and saving, reverse the entry.