When I was reading up on how to build a wheel and properly tension the spokes, I noticed one source explained that the spokes get a slight twist during tensioning. This results in the spokes detensioning by untwisting on the first few rides. To counteract this, you should tighten the nipples to the tension you want, then give it 1/4 more twist, and then back off the 1/4 twist. This unloads the spoke twist, and I think it makes it less likely to work loose.

Of course, before you get to the final tensioning, you should be grabbing four spokes, a pair from each side, in one hand and squeeze strongly to prestress the spoke elbows at the hub. Do that all the way around the wheel a couple of times. Then do your tension procedure. It's apparent that the wheel builder took some short cuts, but you can fix it.