This is not an out of gamut issue, it is a case of really overblown raw channel. It it were a case of out of gamut for sRGB, the spike will disappear when reducing exposure. The more experienced photographers who never miss the correct exposure may have not experienced it, but lousy photographers as myself have experienced it plenty of times, so even if the LR histogram does not tell you the right picture about the raw data (use rawdigger for that). The "Moving spike" is a clear indication of overexposure beyond the capabilities of the highlight reconstruction.
As for the current Process version of LR compared to the old, yes, the highlight reconstruction is invoked automatically, but if you search in the history of LuLa posts by Madmadchan (Erik Chan), who knows a thing or two about LR, explains that the priority of the new process is to maintain the tone of the image, while the previous processes tend to introduce colour shifts.