When transformers are hooked in series they'll draw 5x more power than they should (5kW vs 1kW for the small, 20kW vs 4kW for the large). The issue persists through multiple setup configurations, multiple power draw sources, multiple wires, multiple transformers, and multiple restarts of the game.
Steps to Reproduce
I made a 1.5 min video explaining the issue and showing exactly the circumstances in which it happens. All you need to do is hook the low side of a transformer to the high side of another and check the energy tab for the wire. It's not a display bug either, because it'll break the wires in accordance with these 5x values. https://youtu.be/Q0y73J4151Q
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.