Wow that is a significant difference. If you haven't already, I would pull the tuber out and see what's going on under the surface. Maybe there is no root structure. It also kind of looks like something ate it. But seems odd that they would only eat that one and not the others right next to it.

sometimes the tuber starts to rott and one eye/ bud manages to hang on and keeps growing.

it would be interesting to lift the tuber carefully with a fork or something, and get a closer look

Thank you both! I dug up the tuber today and it was basically a pile of ooze and rot - now I'm really impressed that that little shoot was able to hang on for so long! These are my first tuber based plants and it never occurred to be to check what was going on down there. Constantly learning!

Well if you only lost one out of five, you are way ahead of me! Most of mine turned to dust or mush over the winter. I got one good one out of about 8 that I tried to keep from last year. Dahlias are not easy for me.