There might be wiggle room on that as Yoda looks a tad younger in TLJ, or at least happier.
Luke probably didn’t know what young Anakin even looked like at the time, but Kylo worships his grandfather so either form could appear to him and make more sense than the SE change.
But seeing as Kylo is based on Hayden’s Anakin in many ways, having some kind of CG-Sebatsian-Shaw-voiced-by-Matt-Lanter monstrosity would be lame IMO.
Anakin could make a point about the life he could have lived had he not fallen, going from healthy Sebastian to Sebastian right after the helmet came off would emphasize that. Kylo may have ignored certain aspects of Anakin’s life soley focusing on the Vader aspect.