Might just be a slow finisher🤷 who's the seeds from? I've also grown auto grand daddy purple that had the same structure and it took a while to finish (I didn't find a purple pheno out of them tho😭)
Ya some of the plants I've looked at were huge and had tons of colors for autos but that was the parent lines the devil cream I've found was mainly when it's chopped down.. seed wise tho 2 outta 4 germinated almost like they had thicker shells