Hope this might help someone else. I have 2 of the MK138’s running 2 modified fish tank heaters (2 different tanks of liquid for PCB production). The 2 MK138’s are in the same box and I have mounted the control pots on the box panel. This means that there are cables about 100-150 mm long between the pots and their respective boards. I found that there was an interaction between the 2 boards when each was at around the switching point, which resulted in one board turning off the other’s relay as it switched on. This was without any output connected to the relays and also with independent power supplies. The problem appeared to be caused by the very sensitive nature of the high impedance input to the op-amp connected to the pot wiper. I soldered a 0.33uf ceramic capacitor between the wiper connection on the board and ground which cured the problem. Actually, I think around 0.1uf would do but I had the 0.33’s handy!
As I say might help someone!