I’ve been able to reproduce the issue.
It comes from ForestAdmin’s way of handling null values and empty values the same way. I’ll make a product request to improve this point.
In the meanwhile, dropping (or commenting) the allowNull: false on your model’s definition should let you create and update records flawlessly. It’s not the perfect solution but it will let you work properly while the database will still be able to block null entries and populate the correct empty default value.
I hope it helped.