Emails I've sent to myself don't show up in iPhone Mail

If I use my macOS Mail.app to send emails from myself to myself (from a gmail account to the same account), they show up in macOS, they show up on gmail.com, but they do not show up in my iPhone's Mail app. They can't be found in the Sent nor Inbox folders on the iPhone (they are in both these places on every other mail client). The iPhone still receives all other emails. The iPhone can receive self-emails if I use gmail.com or the iPhone itself to send them. What's causing the problem here? Why don't emails sent from my Mac show up on my iPhone?