Maximum memory bandwith for the Xeon E5-2690 V4 according to Intel is 76.8 GB/s, and there are 4 Memory channels available.
According to Wikipedia, for DDR4 DIMMS, Peak transfer rate for 2133MHz is 17066.67 MB/s, while for 1866MHz it is 14933.33 MB/s
Now, if we consider 1886MHz, then 8 (DIMMS) * 14933 MB/s = 119 GB/s, while 6 (DIMMS) x 14933 MB/s = 89 GB/s. So even considering some overhead, the available memory bandwith is already maxed out, and it would not make any sense clocking the memory higher.
Until you have a maximum of 4 DIMMS, there is 1 DIMM per Memory channel, and you can get more speed. On 1866MHz, 4 (DIMMS) * 14933 MB/s = 59 GB/s, the memory bandwith would not be maxed out, and not even on 2133MHz it is maxed out (4*17066MB/s = 68 GB/s), that's why you could go up to 2400MHz modules.
Unfortunately Intel does not spell out anywhere the exact implications as detailed here above, it is information that is not at all easy to find. As was noted in a comment, Intel's very generic statement (when clicking on the ? next to Memory Types) is:
“Maximum supported memory speed may be lower when populating multiple
DIMMs per channel on products that support multiple memory channels.”
There is some more information available in this thread, and this article about a later generation of Xeons. Furthermore the Intel documentation page on v3 / v4 might be interesting for a deep dive.
Moreover, for a visual guide about the problem of more than 1 DIMM per memory channel, the graphics in this article about 4th Gen Intel Xeon Scalable are interesting to consider.