It is ASUS all core sync overclocking. I do have a negative vcore offset. Would that be the cause?
I have little experience overclocking HEDT on Asus, sorry. But I would attribute this behavior to an odd OC / BIOS setting somewhere yes. Chip instability, unlikely - degradation will cause additional voltage required for a certain clock, not a clockspeed / multiplier drop off. If the voltage is insufficient the OC just falls apart and you'll notice in the usual ways.
You said you had a negative AVX offset, which I know can act funny though I've never seen it drop multi below what its' set at.
What you can do is switch the Asus All core sync OC to a per-core OC, that you just set for 2 ~ 10 core boost at the desired clocks (4.2 Ghz like it is now). This way you can remove any Asus trickery from the equation there while effectively not changing your OC. So you can at least rule it out.
Negative vcore offset, if this is just an adaptive voltage type of vcore setting and not an additional offset value for specific loads, then no, it should not make you drop a multiplier under AVX, rather the CPU will just start giving errors due to lack of voltage or the OC collapses altogether.
Beyond all this, look for any out of the ordinary BIOS settings, I would also look at the safety measures built into it/the CPU such as the maximum allowed Watts during prolonged/sustained loads. You do push that sucker hard and you may exceed a value there to cause it to clock differently over time.