Hi Rob Ainscough,
Thank you for contacting us and for the provided information.
We weren't able to reproduce this behavior using only the provided information. Basically, when we enter a negative value by pressing the minus Numpad key we were able to set or remove the "-" sign. Can you send us additional information about your mask set up? Have you bound the mask to a property from your view model? It will be great if you can send us isolated project from your application reproducing so we can further test it directly on our side.
In addition, the mask which you have set to the NumericInput is invalid. If you want to place zero at the end of the input, you can escape it using the "\" backslash. In your case, the mask will look like this: Mask="###\0". You can take a look at the Mask Tokens
help article in our documentation where the Mask property is further described.
We are looking forward to your reply.
Telerik by Progress