iPhone 6 randomly goes black with a spinner in the center

Since last year, I have been encountering the random black spinner screen on my 64GB iPhone 6 every now and then. When the black spinner screen is out, basically I can't do anything but to wait for the passcode screen to appear. This cycle will probably take around 20-30 seconds. I thought this happened because of my outdated iOS 10.X.

So I finally upgraded to iOS 11.2.6 last week with 15GB of available space. Unfortunately, the black spinner screen is still haunting me! I have got it 3 times this week.

Can anyone help?