This is my first post . . .
Welcome to the Kitz
I've read through your query and what you have asked makes perfect sense. However I can't offer any possible explanation. So let's see what suggestions other members may make.
[slightly off topic]
As an example, I've attached the SNRM plot for Kitz
' G.993.2 circuit. For a long time it has been subject to quite violent oscillations in the SNRM. Then quite unexpectedly, at around 0538 hours this morning, the oscillations ceased.
Try as we might, we do not have any reasonable explanation as to the cause of the oscillations.
[/slightly off topic]