I moved all my emails (13k messages) from Outlook to iCloud a couple of days ago using Outlook 2016 on a Windows computer and Apple's iCloud client for Windows. I see all of my emails in iCloud's web client so I assume that the emails have successfully been moved to iCloud. However, not all emails are being downloaded to iOS' default mail app on both my iPhone SE (running iOS 11 GM) and iPad (running iOS 9.3.5).
The iPad has synced all my emails up to 2013 and my iPhone has synced all mails up to 2014. I'm missing all emails of after that time on my devices (even after I manually pull down to refresh), but they do show up in iCloud's web client. According to iCloud my mailbox size is 773MB and both my devices have plenty of storage space left (>5GB), so that shouldn't be an issue here.
Any idea what I'm missing here? What do I need to do to have both my devices download the remaining messages?