Why was my savings plan debited from my reference account instead of my broker account, and how far in advance should deposits be made to ensure sufficient funds?

I have the following information on the relevant transaction today:

It is set that savings plans are debited from the credit balance of your broker. The amounts for savings plans are already reserved from the broker's credit balance several days in advance.

However, in the case in question, your deposit was only booked after an attempt had been made to reserve the amount for the savings plans.

As there was insufficient credit on your broker at the time the amounts for the savings plans were reserved, direct debit was used - from the reference account registered with us.

Please note: The reservation of the amounts is made several banking days before the execution date of the savings plan. For this reason, we would ask you to make deposits for this at least 5 bank working days before execution.