A mana node that changes hands has its improvement destroyed and turns into raw mana. What seems to be the bug in version 0.25 is that on the screen it still looks like an improved mana node and says so when you mouse over the square, e.g., "chaos mana." In your game as you've described it, you should be able to send a spellcaster to the node and build a chaos mana node.
EDIT: D'oh! Forgot about roading the tile. Thanks, Kael.
Thanks, Noreaster! I had checked the road, it was roaded.
But it seemed even worse thqan you describe. If I recall correctly (hope so!)when I moved my mage there, it wouldn't let me build a chaos node. I could build a DIFFERENT node, I think, I couldn't build a chaos node.
I even entered the worldbuilder, and I couldn't change the tile to a chaos node! I 'solved' the problem in my game by using the worldbuilder to change another node to a chaos node, and I built back the 'first' node with my mage; I do think there is something buggy here.
Best wishes,
Breunor