Thanks for the report, I'll give it a look and see if I can reproduce. Do you remember which defender unit it was? The defender upgrades happen after shopping phase, but before what I call the "planning" phase where you can view ranges before moving your snake. There shouldn't be any situation where the ranges shown while you can move your mouse and see them are different than the ranges experienced during the level.
I'm going to be making another pass on range displays soon, so let me know if you have any other suggestions. I'd like to add some more data to the planning mode like damage from towers and maybe even damage per second on the tile. I could also add a toggle to show all tower ranges at once, although sometimes when you have too many overlapping it gets hard to meaningfully understand.