If the tripping is caused by noise from the card affecting the 12v sense, why the noise from card flows to sense line and why it only affects lower wattage Seasonic models?
The fan should spin when it's not in hybrid mode.
Also, most psus spin a bit on bootup even when it's on hybrid mode (my Corsair does, but IDK about Seasonic, does your other Seasonics spin on bootup?).
so... that might be a fan problem.