Where does the "Mixed Categories" alert come from?
It appears when the same flow contains both marketing and utility templates.
Each template is billed according to its category: in a utility flow, a marketing template is billed at the marketing rate, which is higher. A flow should therefore be 100% marketing or 100% utility.
What if a message in my utility flow isn't approved as utility?
The solution is to replace that template with a Reply node, which has no category and doesn't go through Meta approval.
Keep a first utility template to start the conversation.
Add one or more Quick Reply buttons to it.
Connect each button to a Reply node containing your message.
A Reply node can only be sent if the customer clicked a Quick Reply button (or wrote to you). Without a click, the next message must remain a template.
Benefits of the Reply node:
no Meta approval;
free text that you can edit whenever you want.