When I connect two notes, the connecting line does not appear immediately. Some additional «trigger event» seems to be required to make the line appear.
The line appears once I do something in Scapple, like selecting some other note or moving some note on the current board. But the line also appears once I do something not directly related to Scapple, like making a screenshot.
I have a short video that may better explain what I mean, but it seems I cannot attach any videos here.
I only noticed this issue, when I started using Scapple again recently. Since the Scapple Video Tutorial doesn’t mention this behavior either, I’m assuming it is indeed a bug and not a feature. 
Scapple 1.30.4, macOS 10.15.5