I'm just puzzled why it suddenly took umbrage after holding nicely for over 30 days. A drop and reconnect at the full 6db margin and 70Mb/s would make sense but it seems a bit churlish to have stuck interleaving on.
If you hadn't had any disconnections, then it is reacting to an increase in the error rate. Perhaps a small increase over a long period, or perhaps a few short but large bursts. They are enough to trigger DLM - which is aiming for pretty low error rates.
If it was a one-off, expect DLM to remove interleaving after 2-4 weeks. If it does so, expect the drop+resync to happen again in the 4AM-7AM region.
Sorry it took 10% off your speed. But hey - it looks like you might get an even faster connection now if you wait a bit.
It does look strange, doesn't it. The effect of interleaving does indeed seem to both drop the actual speed and increase the attainable value, while removal of interleaving does the opposite.
Perhaps it is a vagary of the formula used to calculate the max attainable (which is specified in the VDSL2 spec), or perhaps it is a bug in the modem firmware - either calculating the wrong thing, or calculating correctly but displaying the wrong parameter in the wrong place.