I wonder if it's because I'm using modded drivers (having 7 GPUs per board, I had to do the "copy old drivers into new drivers folder"-trick to make it work.
So the only way I can see temps is if I plug in a monitor into the said rigs... which I don't have. My old dummy plugs from 2014 do not fit into the cards either (my dummy plugs are VGA-DVI-I and the cards have DVI-D sockets).
Back to the drawing board.

EDIT: ordered two HDMI dummy plugs. Best thing I can do I suppose.