This issue is nothing as fancy as a game bug, unfortunately. It's due to my attempt to reduce the jank and sudden jumps you could get when going to/from map mode, combined with a slight misunderstanding of which render_modes the zoom variable actually works.
By keeping separate zoom values for each render_mode it is possible to zoom infinitely in the "zoomed into world" mode as well.
I'll have the next version out as soon as I'm happy with the transition from map to zoom into world.