Masha - I am not entirely sure what is going on. You can try adding a couple of cloves of mashed garlic to that soapy water too. That may help a little more.
There are so many bugs and so many new ones coming around it is really hard to say. I have been reading about flea beetles that can do this damage. I also read about earwigs. I didn't have that problem when I planted dahlias but maybe I was lucky.
A few tricks for getting rid of or at least partway eliminating earwigs. They like dark places to hide during the day. They tend to do their eating at night and they leave behind black feces. Take a piece of garden hose and lay it near the base of the plant(s). Early in the morning you can just pick up the hose and shake it into a container with soapy water or you can squish them. Another idea is to take a plastic bottle - pop or water bottle - cut off the top, put a small amount of tuna fishy cat food (or even tuna fish but the experts say the cat food works better) in the bottom of the bottle. Put the part you cut off back on but put it on upside down. Tape it onto the bottle so it stays put. The earwigs go in after the food and can't get back out. Then you can do the same thing, drown in soapy water or squish them.
I don't know if it's too late for your plant. You may just have to wait till fall and lift the tuber after the first frost. If it looks good you can store it over winter and plant it again next spring. Good luck.