Sorry about that, it's not a feature, it's a glitch caused by too much lag. Make sure graphics acceleration is on in your browser's settings. If you're using Firefox, try a Chrome (or another Chromium based browser), as Firefox has issues with Godot web games unfortunately. If that doesn't fix it, the downloadable version should have better performance, making this less likely.
I'd like to look into a way to fix it completely, but it will probably take a while to implement as it would likely be basically redoing the entire way the game spawns notes from scratch, and even then I'm not sure it would work. I'll have another look into what it would take to fix it, but unless it ends up being much easier to fix than I expect, it probably won't be in the next update, since I want to release the Chapter 5 maps update as soon as possible. I'll definitely look into it more after that though!