Ok, figured out what the problem was last night (still playing patch "e", but didn't see it on the "f" update list, so I will assume it hasn't been fixed. When a worker builds a road over an already developed mana node, all nodes belonging to that player will revert to an undeveloped state. Could be exploited to build all the towers with 4 mana nodes...
i just tried to reproduce this on .30e without success. is there something more to it you are failing to mention? using both slaves and regular workers i was unable to affect existing developed mana nodes by simply building a road to a developed mana node. both with the node already started but not completed, and with the node already built before telling the workers to start the road.
are you certain it wasn't the amurite world spell?
what race/civilization were you?
anything else you can think of that you were doing?