I have no idea how I ended up here but I love to see Godot games! Vertex animated textures are something I've been looking into. I've noticed my games performance drops significantly when there are too many skeletal animated enemies on screen.
Here's w hat my zombie 'town' ended up looking like lol. Also why do some zombies run off into the sunset? 
Yeah so in this game there are no skeletons whatsoever! I imagine performance should be pretty snappy. I'll have some YouTube videos up soon explaining exactly how I pulled up VAT - it's not too hard, and super powerful - because aside from instancing, and doing stuff like this, you can also bake out animations from Houdini or Blender.
I'm 99% sure it happens if you alt-tab away from the browser, or if you lose focus on the game window. Maybe I should try to detect that and pause the game simulation. Basically if the game hitches, the simulation logic gets messed up, and the zombies keep moving in a straight line on whatever their vector is.
It doesn't actually happen on the desktop build, so it's probably some WebGL things. The game isn't throwing any errors or warnings, and no run-time crashing, so I genuinely don't know.
Thanks for playing!