RAM Throttling on fully populated Z440

[dayoldy]

Greetings,
I wanna potentially buy a Z440 that has all 8 slots populated with DDR4-2133.
Its RAM speed got throttled down to 1866. The installed CPU is an E5-2697v4. That CPU sports a Max Mem Bandwidth of 76.8 GB/s. And the Total Memory bandwidth from all 8 sticks is a whopping 136.4GB/s (almost double the memory controller's bandwidth! Evidently the HP RAM slot monitors were triggered and reacted somewhat harshly.
But interestingly enough, at 1866 MHz, the bandwidth is still a whopping 120GB/sec. I don't understand the actual logic being played out. I don't see anything about it on this site. The only possible way to stay within the CPU's max if that is an inescapable requirement, is to limit use to 4 slots. 4 DIMMS of DDR4-2400 in quad channel mode would actually be be a perfect match.
Now my Z420's 64GB of DDR3-1600 RAM is populating all 8 slots. The 8 in total, double the bandwidth of its E5-1650v1 CPU as well; yet with no throttling.
So where can I learn about the behavior of both beasts? Is it related to anticipated temps? The Z440 that's throttled down has both memory cooler shroud and front fan installed; but no AIO water cooler. Is it due to the relatively powerful E5-2697v4 and potentially massive temps under high rendering loads?

[NonSequitur777]

@dayoldy,

Yes, your Xeon E5-2697 v4 allows up to 2400 MHz RAM speed, BUT:

The HP Z440 system often defaults to 1866 MHz as a stable baseline, especially when all RAM slots are in use. Another reason could be mismatched or mixed RAM: if you have different speeds, brands, or ranks of RAM, the system slows everything down to the slowest module's speed or a safe default.

Also, a single bad stick can cause instability, leading the system to downclock everything to 1866 MHz to maintain stability. 

Btw, installing more than four DIMMs puts extra strain on the memory controller, often forcing speeds down to 1866 MHz and requiring the specific HP Memory Cooling Shroud.

Kind Regards,

NonSequitur777

[dayoldy]

Thanks for your input.

All sticks are identical. That's what's puzzling. 
If you say that it defaults to 1866 whenever fully populated, that certainly confirms the behavior.
The Memory/VRM cooling shroud is in place, and so is the front fan assy, and stock heavy-duty heat sink. I'm sure the PC has sensed and taken all of these precautionary steps into consideration. 
The bad memory is the only thing that makes sense. 
So what if it's not that then? Is there a way in BIOS to manually overide?

Yes, I get that 8 sticks of any speed within the DDR4 realm can be a workload for the CPU's memory controller. But the HP engineers knew that as well, yet designed 8 slots into a workstation meant for significant cyber work, while not adequately supported by any Haswell or Broadwell single CPU configuration. That doesn't make any sense to me. Even with its default precaution of stepping down the speed, that is still woefully inadequate for users wanting to push it - given the remaining numbers. 

Especially without any communication to users that contains supporting procedures, best practices, cautions, warnings, and troubleshooting.

There must be more here...
It's kind like having taken a freeway off ramp into the desert wilderness, and then finding out that only the first first 5 miles were ever completed..