Maybe the demand for these bikes is so high now, the build quality is starting to drop. As they have so many to build they have to rush through the build to get them all done?
I would say to some degree this is true, I picked one of these up for my girlfriend who wanted to start cycling with me (she chose the bike, not me - she also tried several others including a selection road and road based hybrid's by Bianchi, Marin, Trek and Pinnacle but prefered the Triban in this case) and they were selling them by the dozen! They had about 6-8 of these lined up waiting for assembly for collection, plus some in process of being built up and a few rolling out the door when we went to pick her's up.
You also have to consider the price of the bike, £300..... the wheels are not going to be built like a set of £100+ wheels or a pair of £150 hand built wheels so if they go out of true, re-true them.
Further, cables will stretch fairly notably during the 1st few weeks, unless you pre-stretch them and you may need some adjustment to bring it back in line. A new wheel can also go out of true in a similar way. Not necessarily the fault of
Decathlon.
It could be that they didn't quite get it right though, I am not denying that, they are fallible, as are we all, but it may just be a case of settling in.