It appears that Forms doesn't display the manual text input for signatures when it detects a mobile device.
Unfortunately this also happens on Chrome and Firefox browser running on touch enabled laptops like Surface Books and Surface Pros (See screenshot at the end).
We tried it on multiple touch enabled devices and had the same result on all of them.
Is there an option to force it to display the manual text field? Possibly via css?
Regular Laptop:
On a Surface Book: