65 NU7100 for 3999 and free rm350 Petronas voucher. Ok deal?

The primary difference is , you are paying extra for HDR Performance along with some advanced TV features like Variable Refresh Rates ( V-sync for gaming ) & cable management .

An important thing to know ,  despite the current entry levels 4k TV such as NU7100 , XF75, XF70 being advertised HDR TVs , they are not actually considered as HDR compatible . They are merely able to digest the signals , they don't have the hardware specs to display it .
On the other hand , NU8000 is a much brighter TV . It is able to utilize the local dimming to show bright highlights from HDR content so the picture can , to a certain extend "pop" out of screen. The TV also supports a wider color space coverage in both rec 2020 and DCI-P3 , which are what most modern 4k UHD movies are mastered with.

Obviously if you are looking for true HDR experience , neither NU8000 or NU7100 are an ideal choice . The latter doesn't quite nearly has the hardware specs to support it .  The former is not very good at it either . Since edge-lit based local dimming will brighten up the entire portion of the picture when showing bright objects , which effectively reduce the dynamic range of the picture . 
Nevertheless , among the sea of mid range TV models in MY ,  it is the only one which can give you a taste of what HDR is like without forcing you to sell your blood and kidneys.

I noticed this too for 55 NU8000. It seems to auto adjust the brightness or something based on how bright is the scenes/environment?
The not so nice thing is it were dimmed to the point that it looks dark and greyish.

Is there some sort of settings we could tune to reduce or prevent this from happening?

Is there a term for this?

Let's say the scene is a bright sky (or just total white even), and the camera pans around. Can see noticeably some parts are "gray-ish" as if there is a layer of dirt on the screen (this is the best I can describe), making some areas on the screen seem darker than the rest on the same scene but only when the camera pans around.

Hoping it's normal and not indication of any defect (my TV - LG 65SJ850T)

Thanks. Other shops also run out of stock. so maybe i'll wait until next stock in.