I tried running with the exact data you gave, and it works as expected when saving with value '0' instead of null in both DB or selecting with DAL. The Version is shopware 6.6.8.2.

I can't find exactly if there is a ticket fix issue for this or not. I suggest you upgrade to the latest version, currently 6.6.9.0, and try again. If you still have the problem, try investigating whether installed plugins are affected.