When you send a preview of a myflow newsletter, to yourself or to another email address, personalization tags such as [customer_firstname] and [customer_company] are not filled in. They appear as plain text, for example [customer_firstname]. Adding yourself as a contact in myflow Contacts doesn't change this.

The tags are filled in when the newsletter is actually sent to your contacts, using each recipient's details from their contact card. See Personalize your automations and newsletters with tags.