Copy the formula down, and it works. Each balance is the balance above it plus the amount on its own row.
Now you find a receipt from January 11 and insert a row for it, between rows 3 and 4. The new row has no formula, so you type one in, or copy it from above. But the balance below it now says =C3+B5. When the rows shifted, the reference was adjusted to keep pointing at the same cell, and that cell is no longer the row above. The new transaction is skipped without showing an error of any kind. The balance is just wrong from that row on.
Delete a row instead, and the formula below it becomes #REF!, and so does every balance after it.
None of this is a bug. To a traditional spreadsheet, “the previous transaction” means “the cell above”. The two agree only as long as nobody rearranges anything.
