Sorry to Necro this thread but it came up when I was looking for the same thing. I have definitely seen the resolution below to this issue in other threads.
Dragging the full CN from the finder to create a backend group does not work (pretty sure this is still a bug 5 years later). Instead you need to use the straight CN for the group, in quotes when you assign targets to the policy.
For example, I have a global security group nested under other AD OUs, and use the simple CN with quotes "CN=Internet Access Group" - when inserting a group into the filter (create a new group). AD will only let you define a single CN of the same name anyway. Works with the UTM's web filter. But it doesn't create a group in "Users & Groups" as you would expect it to... it simply maps correctly.
This article is related but overly complicated to my mind: https://community.sophos.com/kb/en-us/120658
...although it probably describes the issue in full.