Cut worms cut through stems. They don’t rot tubers. I think the ‘worms’ have probably colonised tubers which are rotting for some other reason such as a fungus. What is your watering regime?

A quick question. Have you worked in homemade composted soil which contains some kitchen leftovers into this garden bed over the past three years? I would appreciate an answer as it may help a research project I am working on.

How were they not doing well? Were they turning brown? Were they drooping?

The rotted dahlia tubers look more like a water or bacterial issue than a worm issue. What kind and what is happening in the soil is the first avenue that should be explored. You only mentioned the dahlias demise. Are the other plants doing ok or are their roots rotting too?