Let us know if you can reproduce it. A lot is still in in flux between versions so it’s important to know if a bug is happening on a current build and how we can recreate it.
I managed to reproduce it, but unfortunately I forgot how to take screenshots with steam. (yes never did that before) and now I can't seem to find the map where it happened, as I have to do some earlier missions again to reach that map where it happened.
I found another issue though, where the upgrade points can be below zero, and probably allow me to spend endless amounts of them on the towers. But then the game complains about core spending limit of 6.
So I'm not sure if it was a design decision to allow spending points that can't be actually used, but it's kind of confusing.
So either inform the player before he spends points that can't be actually used before other points are removed. Or don't allow spending them in the first place.