I don’t think that this is related, but please go ahead and prove me wrong 
In this thread post I provided a modified interfaces.py that can be put to the~/local structure and
overshadows the factory version of that interfaces check.
It is just extended to log the raw rx/tx octest it got from the device and some related, calculated vars.
https://forum.checkmk.com/t/weird-spikes-on-interfaces/39480/14
Perhaps it is possible for you to use it on a test instance of your checkmk for the device ?
If you have question please do not hesitate to contact my by private message.
My interest in this topic is to mythbuster that checkmk does it wrong.