I’ve never tried using sign on the left, but I suspect that it’s not allowed to be entered in this way. I think you either have to enter the sign on the right or press the field- key to enter your value as a negative. The definition of your field with a minus sign on the left is for field <b>editing</b> (display purposes) only, not for field entry.
Again, I’m not entirely sure that this is the case, maybe someone else here can clarify.
As Martin wrote, you can not use LEFT sign in an input field, but field- to enter it into the database file.
Only – ugly solution – use a character field and edit it in the program – then you could find the – at either end of the string, check for properly formed digit value and convert to numeric.