Steps:

1. Go to Content/Content structure, click Create and select Form
2. Fill the title and click Form edit button
3. Drag and drop for example Number field (it can be also Single line input) and button, publish the form.
4. Go to front office to see the form. Type some numbers in Number field and click the button - works fine.
5. Go to Content/Content structure again, click Create and select Form
6. Fill the title and click Form edit button
7. Drag and drop again Number/Single line input field and button
8. Click Number Settings button, do not change anything, click Submit and publish the form
9. Go to front office to see the form. Type some numbers in Number field and click the button

Actual result:

Validation for the Number field is displayed.