Send email when order updated

I want to set up a rule to send a custom email when an order is saved. I am using the Order Saved trigger, and plan to use the value of a yes/no ACF field as a filter, to decide whether to send the email. In some cases (including my testing), the only change made to the order would be to set the ACF field.

However, when I save an order no email is sent. The logs say:

Workflow started object: Automattic\WooCommerce\Admin\Overrides\Order(590)
Workflow . Filter processing returned false

590 is my test order number. This happens even when there is no filter.

Can you suggest what could be going wrong here?

Thanks in advance.