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?
Edit: spell checker option is not in the setting anymore! It has been moved to Samsung keyboard, is that a bug?
i.e. I can't do this:
Settings > system > language and input > advanced > spell checker > language > set English instead of system default.