Probably a mixture of the clay retaining water/ not draining and the shallow bed. I used cavity/ cinder blocks to build up the bed, much cheaper than railway sleeper style beds. Originally had to hand till the soil with a pitchfork down a few feet to help break it up, before planting.

Was the soil particularly wet or over watered? Sometimes if it's too moist they rot or not well drained (grown in a bag). Potatoes need deep soil that's loose, usually tilled. Or in a raised bed if in a hobby garden. Weird looking ones probably hit hard soil or a rock or something. Baby potatoes/ new potatoes I think are usually 1st sprout, still delicious especially with butter. Keep at it. Replant some that you don't eat. Try and give the soil a rest or leave a patch empty to rotate. Stops the soil losing nutrients and solanine build up, which is when they turn green due to light exposure, from shallow soil or rain/ water washing the soil off. Make sure to build the soil up around the plant continuously as they sprout, to support the stems. Keep at it! 🥔💪