I’m unfortunately not able to recreate this issue. I created a person or group field named 1.1 and added it to a DFFS form and did not get any errors.
In general, I suggest that you create field with a name without special characters initially (only use a-z) and then rename the fields to whatever display name you want. Also always use fieldnames longer than 3 characters and not start the field name with a number. This makes the internal name of the field easier to identify and also easier if you want to use custom js to address a field.
Alexander