Email Stuck Loading in iOS Native Mail App for Exchange Account

Hi Everyone. Around 3 weeks ago iOS users in my company started reporting an issue where email in the native mail app would display "loading" in the body of the email when they would try to open it. For some users the issue is sporadic, for others it seems to happen pretty consistently. The problem only affects the corporate Exchange account we setup on the device, not any personal accounts. I went through the list of steps that Apple recommends for this issue (remove/readd account, reset network settings, updated OS, etc) but none of these help. The problem also follows affected users to other iOS devices (even brand new devices). If I setup a non-affected user on an affected iOS device they do not see the issue, which tells me this is not a device issue but instead something with Exchange. Out of 20K iOS users in our company, only 50 or so have reported this problem over the past 3 weeks.

I suspect the issue is the email is being truncated at some point. Our ActiveSync policies set the MaxEmailTruncationSize to unlimited, which have been set this way for over 2 years with no issues. As a test, I sent a working and non-working user the same email then had them turn on Airplane mode when the email appeared in the Inbox (without opening it). After Airplane mode was on and they confirmed no connectivity to the internet, I had them open the test email. For the working user the full email displayed when opened. For the non-working user they received the message "this message has not been downloaded from the server" when they opened that same email.

Microsoft states that EXO only does what the client tells it do and does not truncate the response on it's own. I go back to the fact that the problem follows the user to different iOS devices. All our users have the same EAS policy assigned, so there should be no difference there. The affected users have different size mailboxes too - some rather small, some on the larger side. Also, problem has not been reported from our users in O365 dedicated environment - it only comes up after their mailbox has been migrated to EXO. Additionally, for some users the issue went away when the Exchange mailbox was moved to a different database.

Has anyone seen a similar issue crop up recently? I'm starting to run out of ideas here. Our firm does not have a support account with Apple, and even if we did I have a feeling Apple would direct me back to Microsoft on this one.

Thanks in advance!