Why would one battery slot have springs on BOTH ends?

No soldering needed, I promise. I had the same thing happen with a puck light from a clearance bin a couple years back, except mine had one slot with double springs and another with none, pretty much what you're describing. I almost chucked it before I pried the battery door off and looked closer. On these cheap lights the spring contacts aren't soldered to anything. They're just flat metal tabs that sit in little slots molded into the plastic, and the whole assembly snaps together at the factory. Someone on the line pushed the tabs into the wrong holes, which happens way more often than you'd think when the plastic parts look identical from the outside.

Here's what worked for me. Take the light apart, ussually a couple of small screws under the sticker or in the battery compartment, or it just snaps apart if you work a flathead around the seam. Once it's open, look at the back side of the contact plate. The metal tabs lift right out of thier slots with a fingernail or small screwdriver. Pull the two tabs that are wrong, swap them into the empty slots, and flip whichever ones need the spring facing the right direction. On mine the tabs were symmetric, so orientation was just a matter of which way the spring pointed.

Before you close it up, drop the batteries in and press the lens a few times to confirm it stays lit. These things cut out the second anything loses contact pressure, which is exactly the flickering you're seeing now, since the wedged batteries are making intermittent contact. And honestly, don't beat yourself up over it. For what these cost, one in three being misassembled is about the expected quality rate. The fix takes ten minutes and you keep the light out of the trash, which beats returning it anyway.

I've seen this same issue with a cheap motion sensor for my garage. The casing plastic is usually really brittle, so when you get it open to swap those contact points, just be gentle not to crack the housing. Once you get that spring tension right, those little electrical connections hold up better than the surrounding plastic often does. It really just shows how quickly those cheap molded parts fail. Keep those tabs clean when you handle them, too, just in case there's any residue.

It is definatly a manufacturing variance issue with those mass-produced puck lights. In my time dealing with similar consumer electronics, the issue is usually the plastic housing tolerance rather than the spring itself. Most of those low-cost plastic contact assemblies are molded with a clearance of around 0.5mm for the tab seating depth. If the tab isn't fully seated within that tolerance, it won't maintain consistent pressure when the battery is pushed in. Try pressing the contacts with the batteries alredy seated to force the spring fully into the slot before you finalize the swap.

Update: took the poster above's advice and pried the battery door off before doing anything drastic. Turns out one spring had just popped loose from its little tab and slid down into the empty slot next to it. That explains the double spring and the missing one. Pushed it back into place with a screwdriver tip and the light works fine now. My only worry is it popping loose again every time I change the batteries.