The screenshot was taken with no control inputs. The thumbnail was taken a few seconds later while pitching up hard (S key). The elevons move as expected but the wing vectors (green lines) show an oversimplification. When pitching up, the bulk of the wing should provide lift, only the pitch elevons should make downforce. Instead, we see wing and elevon vectors mixed together to make the whole wing create downforce in the wrong position.
This isn't serious; builders could use multiple wings to give the impression of a delta wing, but the fix seems simple to me. It could be fixed with separate lift vectors for each part of the wing; one for each elevon and one for the non-moving part of the wing.