I think chronically online people like ourselves are experiencing things a lot differently than average offline fans. Like I am pretty sure things you read online affect your own views, things like the rap part in Set Me Free or the charting discourse subconsiously affect your own perception of the song.
Again, it is not the best song, it is not a very catchy one either, but it is a decent song. Most people won't notice there is something wrong with the rap unless they go online and see people talking about it. Most people don't know what's happening on the charts either.
Besides, things ain't even half as bad as stan folks say. This was a pretty experimental comeback roll out. Pre release single and then releasing Korean and English versions of title song at the same time while playlisting the latter one. I am sure there was something people at JYP were trying to achieve with this. Whether they managed or not, only they know. They will be evaluating all the data the have in hand and learn from it.
I shade JYP the most for the creative choices they make, however you gotta admit, they are doing something right if Twice went from "would never do well in the US" to becoming the best selling and most streamed girl group in the country.