The yellow patch (I supposed) could be mosquito bite
. Since you mentioned the root was fine, I think the plant should be OK.
Tap water in Malaysia is fine for CPs, I used tap water for many years already.
All my VFTs are under full sun, they don't seems to decline but infact enjoyed it. I did hear that temperature in Northern Malaysia is higher compared to southern region, something like 41 to 43 degree C if I'm not mistaken in the extreme hot evening
, that could be too hot for VFTs which shade will be needed. In KL, at least in where I lived, the tempeature seldom go up to 38 degree C. I never place VFTs under shade cloth before, maybe should make a try, who knows they show some surprises
!
From my observation, CPs don't like constant temperature throughout day & night. They eventually decline although they show sign of OK in the initial stage, night time cooling is a must at least to mimic their natural habitat.