Quick update! I have now found out why this occurs.
It seems to occur when the game's viewport is not at the correct resolution (most likely occuring if the webpage is zoomed in / out) - so the player will unfortunately have to manually adjust the zoom on their end. 😔😔
// as a side note, I now have a fix for it in engine, I just needed to call a different function to get the adjusted size of the viewport, but alas submissions are over