Neeed to backport these:
Bug 752993 - 1.0.4: Interface enp3s0 has MTU of 1492 -- should be 1500 (edit)
Bug 752508 - NetworkManager 1.0.4 minimizes MTU
Verified by setting MTU to 666 that should be ignored and set at least to 1280 so ipv6 can work. Setting MTU to 9000 works as well. Verified on all architectures.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.