Seasonic X was indeed better built, even compared to Prime but that's not exactly the problem here.
What's your motherboard ? Are you using stock cables ?
The problem i'm referring to is that Prime based PSUs shutdown with RTX3080/3090 pretty much regardless of wattage, there's a ton of reports all over the internet, i've seen like at least half a dozen of them on Reddit in the past month. JonnyGURU also confirmed it with Corsair AX which is Seasonic Prime TX. It's not OCP tripping on Ampere power draw spikes (because the PSU doesn't latch off) but a design oversight, the GPU feed back some noise on 12V rail which the PSU doesn't react very well to. Some high-end motherboards (ASUS Crosshair / some -[insert random letter] series, Gigabyte Master / Designare) have been seen to remedy this issue somehow, maybe they have some filtering between the PCIe slot and 24-pin cable 12V planes.