I can honestly say, I've not seen this before, only in revegging plants.
I think its just a dud mate. Its not pushing onto the next phase.
You could keep trying. But, Idk if it'll ever fatten up to what you'd expect. Its a weird one. Autos at times can be very fickle. And, go their own way Its why id never just do 1 grow, as what do you do, when something goes wrong. And with autos, its not if, but when