If using the numbers appearance type and using a mobile device, the cursor does not always appear in the field. Despite this, the user is still able to enter the numbers in the field using the numbers keypad, but the values are not saved when saving the survey. If you select the field twice and ensure there is a cursor in the field before entering your values, the numbers will be saved.
It's difficult to explain or show in a snip, but I've attached two snips, the first (capture1.png) shows no cursor, yet the numbers keypad is visible and you can enter the numbers using the keypad into the field (but they won't be saved). The second snip (capture2.png) shows the cursor visible in the field, and also the phone's native keypad, if numbers are entered in this state, they will be saved.
When using the desktop version clicking in the field with your mouse or using the tab key to jump between fields always seems to produce the cursor in the field so it doesn't seem to be an issue.
If this is not a bug then it is certainly something that users need to be aware of!.