Not sure what would happen if urchin poked it. Don't you have a tuxedo urchin? Can't see that piercing a nem.
Not saying this is the issue but I think you're feeding too much. A nem that size can go with a small meal once a week tops. If it deflates multiple times per day, I would not be surprised if it started walking soon. Is it happening more than once per day?
Another option is that it may split. While cool, it may be an indicator of stress (survival mechanism) if it splits so soon after being in your tank. I would run some water tests.