On Samsung phones, the decimal separator and minus sign are in the same keyboard button and you press it once for the decimal sign to be written and press it twice for the minus sign to be written. After some tests, I found that this works fine on cultures that use "." as the decimal separator, but not on cultures that use "," (like Portuguese).

 

What happens is that no matter how many times you press the ", / -" button, it never writes the minus sign, therefore doesn't allow writing negative numbers.