Unreliable switch on string of LED lights
It sounds like the original system design is the problem.

If that wall wart is outputting a nominal 3.2v constant voltage into white LEDs, then it's asking for trouble.

LEDs should be current driven, they'll decide the terminal voltage for themselves, which will be in the 3.2v ballpark. However, the terminal voltage can vary up and down a little with temperature. Even without the switch, you might expect the LEDs to be very bright, or very dim, as conditions change slightly.

What the power supply ought to be doing is delivering a constant current, into a voltage that can vary significantly from 3.2v. A constant current output would make any small change in switch resistance completely irrelevant.

All switches exhibit some instability of contact resistance. However, you might expect a mains-rated switch to be worse than a specialist low voltage switch in that regard, as the contacts are designed to avoid damage when switching energetic circuits, rather than have a good stable low resistance.

It's quite possible that both switches are 'good' in the sense that they'd meet their specifications when used appropriately. This use between a constant voltage power supply and LEDs is completely inappropriate, the switch resistance has an inordinate influence over the current flowing.