I have several in my house and we've never had the problem you describe. I did have the opportunity to reassemble the mechanism once when the hinge pin became disconnected. I'm wondering if somehow the mechanism is installed backwards so that it's pushing the seat down instead of fighting the weight to keep it up (hence soft close).. Does it still soft close? You might take a look at it if your somewhat handy, it's pretty easy to disassemble.

Yes, it soft closes.....it just won't stay upright unless you hold it in place for about 30-45 seconds. The lid always stays up fine but the seat is the problematic part. We had the same exact problem with both seats we installed.

Unlikely as it sounds, I'd check the toilet for level. If the front is a bit lower than it ought to be, the lid would be more likely to stay up, and the seat to fall.

Or the back of the toilet's too far forward. We've had 2 toilets with that problem. Not soft close, but regular seats - the lid would stay up, but the seat wouldn't. Ever.