And who is going to void your warranty? The warranty police? Are they going to hire forensic scientists to open the uke up and dust for the buyer's fingerprints?
In my experience with instruments of such quality, a replacement won't be an improvement. With a company that mass produces cheap instruments like Stagg, the warranty is kinda pointless. You could go through 50 of the same model and they will all have the same problem. I have been down the warranty road and it has almost always resulted in further frustration. This is because it's a problem with the production quality itself.
I have since fixed many ukes and guitars by replacing the cheap stock piezo that it came with. I have managed to improve instruments like the cheap Stagg by investing the $7-10 into a new piezo unit and a little bit of my time. To me, it has always been a worthy investment, and very educational too. The core problem with crappy electronics is often the bad quality piezo that they use. If you replace it for the same model, you will end up with the same piezo, just as bad. If you put in your own, better piezo, you will end up with a customised and higher quality instrument. Changing pickups on electric guitars is a very common practice.
Besides, it won't even void your warranty. Unless you run into the Federal Bureau of Warranty (which doesn't exist by the way), I have always found that dealers are happy to offer you a replacement or refund with no questions asked. Instruments stores have no interest on what is actually wrong inside the unit because faults such as this are so darn common.
Of course, the biggest lesson learnt is to save money for better quality instruments in the first place
If you can get a full refund, then go ahead. Save that money and get a quality electric ukulele, such as a Godin or a Pono!