OneDrive for Business issue when upgrading from 2.5.2 to 2.6.0

[LDWme]

Hello,
Over the last few months, I've been trying to upgrade my pfsense box to 2.6.0. Whenever I upgrade all the OneDrive for business devices in my network seem to stop uploading files or at least start failing 90% of the time taking forever to upload a small 10MB file when they do.

I've tried multiple things to fix but the only reliable fix I can find is reverting back to 2.5.2, is anyone able to assist or had this issue or have any idea what might be causing it as I'm starting to run out of ideas?

So far I've:

• Looked through pfsense logs for any issues.
• Looked at the firewall logs for traffic being blocked.
• Looked at pfsense settings for IPv6 to see if that was causing issues.
• Tired of removing PiHole from my network and setting DNS manually on clients to CloudFlare or Google.
• Reset my iPhones network settings
• Ran packet traces from both my iPhone and affected PC but traffic seems to flow between OneDrive (Azure) and pfsense to my phone fine.
  I did notice some "ICMP 204.79.197.200 unreachable - need to frag (mtu 1492), length 576" byt my MTU is set to 1500. I don't really understand MTU very well honestly.

So now running out of ideas I'm hoping someone here can help or has had this themselves.
Thanks in advance for any comments.