I can attest to this as well. It does seem to vary based on the frame. I've tested it to see if it was affected by stats, attachments, colors, and gear/weapons. None of these seem to be affecting variables. Furthermore, it seems that only a few select frames--Nidus, Baruuk, Wukong Deluxe (that I could find so far)--can withhold the Spore Ephemera shimmer effect across 90-100% of the warframe. I would guess it might be an issue to how the shimmer "skin" part is wrapping around the frame, or with polygons/textures etc., but I also do not think that this is intentional (for extreme example of it covering less than half the frame: Hildryn). I hope this possible issue (if the partial/inconsistent covering was not originally intended) is resolved.