Yes. Right now it's not a bug. While creating this fueature I've asked users how this should work. Almost all of them suggests to not include costs in the past automatically.
Right now I'm not sure about it and I'm thinking about asking users for this. You know - do you want to add missing costs to current date.
If you need, the simplest way now is to use csv to add previous costs or add items in the past as one time items.
I have same problem right now. I entered road taxes from date 2015/07/23. 20€ each month. The list will stay say it is 20€ at all time. But that's not right for sure.
I can enter each month on its own but that's a lot of work till 2015.
Like below. To add it, you can use CSV file. Add one item and in text editor copy this. On my todo list is still change for this. Right now - this is not a bug, but I'll ask user if he wants to add multiple items or just start counting new month.
Customer support service by UserEcho