Curious line spacing changes

So I fiddled around with my test project and stumbled upon a curious bug. I’ve managed to recreate it ONCE, but after clicking the spacing options it miraculously fixed itself. Seems to be an updating issue with the editor. Well, here goes:
This is how the lines started:

I then marked and dragged sentence no. 3 up behind sentence no 1:

The 1.5 spacing seemed to vanish between sentence 1 and 2 - until I clicked the spacing menu up top. That fixed everything and I can’t reproduce the bug anymore.