Not sure about if this is a bug or intentional design. The separators below the title bar are still not consistent. The left one seems to be the old look. In single column sidebar view this has been changed to the new look ( thin line that does not touch the horizontal line below), which is what the right one looks like. It should be understandable with the screenshot provided. The two separators have been circled.
The plasma version is 6.1.4.
