It only happens when your cursor is in the exact position as orphea is:
https://gfycat.com/pl/bothdelayeddungenesscrab
something similar happens with Zagara’s tumors on Towers of Doom, but only in the bottom lane close to the front walls, results in flashing creep and other visual errors, it happens every so often but hard to reproduce
Best guess would be it has something to do with specific terrain the spell is cast on, similar issue would be with Kaelthas on Sky Temple where his Flamestrike is sometimes barely or not visable at all, depending on graphic settings