Swiftkey underline not showing

I am using Samsung Galaxy S6 with Android 7.0, and here is the thing:

When I use Swiftkey, the red underline is missing.

When I switch back to Samsung keyboard, the red underline is working again!

So, what is the problem? I remember that red underline is a spell checker feature included in the core Android OS to mark the misspelled words,and previously it was working with Swiftkey.

Can anyone assist me, or if you are a Swiftkey user on Android please tell me if you got the red underline or not?