Hi.
Since iOS 13, Apple does not provide a way to enter negative numbers on a numeric keyboard on iPhones. We are not sure about iPads since the only one we have is stuck on iOS 12.5.
To overcome this issue, you can use a TEXT question instead of a numeric one.
You could go the extra step and add a regex to allow only numbers, the minus sign and the dot →
^\-?[0-9]*\.?[0-9]+$
