What happens when transactions are in different accounts, when the budget is only on 1 account?
At the moment, a single budget event can only be applied to a single account. This is so that the balance of the account can be forecasted.
If you have transactions that are categorized towards this budget, but they are not in the budgeted account but in other accounts, they will still be assigned to this budget if it falls within the budget period.
The transactions don't need to be in the same account as the budget event, for the transactions to be assigned to the budget.
If the accounts are similar, you may want to group them so that you can apply a budget event to all of the accounts. You may want to check the article: Everything you need to know about grouping accounts