Mail app never syncs automatically

Despite setting a sync schedule for my my account in the mail app, it has never performed a sync automatically. New mail is only ever fetched when I open the app, and pull down to refresh. When I open the app, the "Last synced" note also shows the time I last synced it myself, which might be a whole day ago. Using the stock mail app on a Galaxy A5 2017, though this also happened on my Galaxy S6. The account is using IMAP. I've tried deleting and re-adding the account and it doesn't help.