iOS Mail thread generating new Message ID

I have a mixed environment that uses Exchange 2010. Clients use Outlook (on PCs), Mail (on OSX) and Mail on iPads and iPhones.

I have found Email threads to be a little messy with iOS 10 using the Mail app, but for the most part, it works OK. The issue is that iOS seems to break the Email threads - Sometimes!

I have several iPads Airs running iOS 10.3.2 and utilising the Mail app for emails

As messages are replied or forwarded, the email usually maintains the Message ID and keeps all emails in the same thread. Occasionally though, and mostly when the email is forwarded on, the threading breaks and a new Message ID is generated (I can check this using Exchange).

I have not been able to find any evidence or support as to why this is happening.

Is this a known issue?

Does the iPad's Mail app handle threads differently to Exchange? ie subject, sender and recipients only (a change in any of these could possibly disrupt the iPad's ability to recognize the existing thread and force a new Message ID to be generated???)

Can anyone even give me any insight here?

I'm left guessing at this point. Any insight would be appreciated.

PS. Sorry for the novel