Token renders in message preview and test message, not in final sent message

I'm on CiviCRM 5.35.1 with WordPress 5.7. Today I used CiviMail for the first time to send a mailing to a group of contacts. I used some tokens (custom contact fields) that rendered fine in my "preview HTML" view and in the generated message when I sent a test email to myself. However, in the final sent message that went out to recipients, they were not rendered and instead displayed as {contact.custom_10} and {contact.custom_9}. Other tokens were rendered just fine. The only thing that seems different about these two is that they are custom fields, and that they were in a bolded line of text.

Is this something I've missed in our configuration? Let me know if you need any other information. Thank you.