In this clip, circle A shows a black vertical line separating two fields and providing a visual point of reference to grab and change the field size.
Oval B shows that the last field displayed lacks that separator. Since the field is the last one displayed that black line is not needed to separate the field from the next one, but it is still needed to provide a guide to grabbing the field's boundary so it can be enlarged or compressed.
Thanks for your post.
I will open a ticket that we should look into this.