Generic Plug-In and H.264/AVC: levels mapping weirdness

I noticed the following weirdness when AMA-linking H.264/AVC (i-Frame MOV or long GOP MP4) files from a Canon EOS 80D camera with the Avid Generic Plug-In: the clips get automatically a "Levels scaling (full range to video levels)" Color transformation, which isn't too surprising as the source files have "Color range: full" as revealed by MediaInfo. BUT when looking at the resulting RGB histogram (be it in the Color Encoding tab of the clip's Source Settings, or in the color correction mode) there are very obvious "comb" spikes in each color histogram (as a result of the video range compression ?) denoting a not-so-smooth spreading of the levels across the range.
In pre-8.9 versions (or when forcing the QT plug-in in 8.9), the same clips get automatically the proper range mapping (15-235) but the RGB histogram is clean and smooth, unlike for the Generic Plug-In + automatic Color Adapter. I understand the QT plug-in takes care of range scaling "internally", as there is no Color Adapter needed in this case.

Isn't the presence of "comb spikes" in the histogram the symptom of a problem with levels scaling when using the new method (Generic Plug-In "AVC long GOP 6" + Levels Scaling Color transformation) ?
Should I simply continue working the old way, by forcing the QT plug-in in the Source Browser ? It looks like the QT plug-in does a better job at evenly mapping full-scale to video levels [for this kind of clip format].
I recknon this camera model/family is pretty common, so I guess it shouldn't be difficult to find someone to confirm this observation/reasoning.

I tried this with 8.9.2 on 2 different computers, and also compared with 8.8.5