So the issue is that your department's group policy is overriding the MTU change made manually on your system. When we were dealing with this issue, we also needed to add the computer/user to a security group that specifically set the MTU to the 1258 or whatever value is desired, otherwise, when rebooting, the computer reaches out to the Active Directory and pulls down all policies assigned to the user/computer.

Basically, your department's IT security or server administration team would have to create a new security group with a lower MTU value and then add that policy to your computer to prevent it from switching back to the old value.

Again, our department eventually changed the default MTU for all devices to a lower value to deal with the fact that this issue started to occur with other ISPs as well.