Borders around edit inline items seem wrong. Inline items get their own dark header bar but nothing at the end of the item so it doesn't appear separate from the next property in the parent object.
At the very least I think it needs a rounded edge and a strip of light grey between the inline item and the next property.
Ideally it would also slightly indent the whole inline item so it looks like a "sub-item". It already indents the expander arrow to show this relationship. The bordering should be linked to this.
This test class code repro'd the issue in the property editor:
class UQATestItem : public UObject
class UQATestSet : public UObject
class AQATestClass : public AActor
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-16663 in the post.