But I was wondering is there anything ambiguous when I've passed a number literal in brackets such as in this case?
Probably not, but it would have made the grammar more complex. Default member initializers were made to support only uniform initialization and copy initialization, because those things couldn't have appeared in existing code. And differentiating them from function declarations is easy.
Adding another overloaded usage for parentheses was never a goal. It's overused as is, IMO. If it's not supported in general, supporting it just for this particular use case you have in mind is hardly a useful feature that would merit the grammar contortions.
