Moodle 3.4.9 (Build: 20190513)
Dataform: 3.3.2 (2017051502)

Steps to reproduce:
1. create a dataform activity on a course page.

2. add file field to your form / entry template, make sure it is using an '*' in front of the field name to make it required:
ex: *my_certificate

3. create a default view

4. save and add a new entry - notice that the file field does not show/display

Also - I searched and found a couple mentions of this but no real bug report started:
https://moodle.org/mod/forum/discuss.php?d=261060#p1131047