Yes, something on your board is likely disabling Turbo Boost when you are pushing your setup to the limit. This can be triggered by heat or power consumption. It doesn't have to be the temperature of your CPU which is still fine. It can be the temperature of some other sensor on your board but I am guessing that this throttling is being triggered by power consumption. If this is the case, adding more voltage will increase power consumption and will likely trigger this throttling sooner.
As a CPU heats up during a stress test, power consumption tends to creep up. You can run ThrottleStop in monitoring mode with the Log File option checked so you have a clearer understanding of what the trigger point is. Keep an eye on the power consumption reading while stress testing. It might be based on that.
Asus used to do the exact same thing when the first Core i CPUs came out. They limited power consumption by disabling Turbo Boost to make sure nothing on the board would go up in smoke when doing some extreme overclocking. They were nice enough back then to release some modified bios versions for enthusiasts that are more interested in ultimate performance and not the long term durability of their motherboards.