After searching in people reported bugs and iXsystems Community forum, It seems like I have a unique issue. I've found people said that their HDD temperature doesn't show up if they have more than one pool (which will fix in v.11.3). Or some people reporting their HDD temperature show up with a delay. Other people were saying they had HDD Temp issue before v.11.2 U3, but my problem is none of them.
I've actually had HDD temperature in Dashboard and Reporting UI when I've had upgraded from v.11.1 to v.11.2 and I had it until v.11.2 U3 (even though with a noticeable delay. But after v.11.2 U3, it's vanished and never showed up again. I'm currently on v.11.2 U6 and still no sigh of HDD temperature in the UI.
The S.M.A.R.T service is enabled, and I can see the temperature of my hard drives in CLI:
Please advise if you need me any logfile or debug info to provide.