From discussion https://moodle.org/mod/forum/discuss.php?d=260189#p1127748:
I have problems using "Separate groups" in dataform.
First of all, I create a dataform and in "Common module settings" I select "Group mode: Separate groups".
Generally, there are two groups in my course: groupA and groupB.
Then, I select "all participants" and add a new entry in dataform. This entry must be visible by everybody. Right?
Then, I select groupA and groupB and add an entry for each group.
Now i have three entries: one for all participants, one for groupA and one for groupB.
With group mode set in "separate groups' participants can see only entries for their group but cannot see entries for all participants, as they should! For example, a student that belongs to groupA can see only groupA's antries but not all participants' entries.
Also, if I use a field of type picture or file, on this group mode, files are not displayed for students or not_editing_teachers. It seems like file to be corrupted ().
This automatically changes if i change group mode to "No groups".
I have also to notice that I don't use group entries selection. ( Actually, I have also tried to use it but with the same result)