Spell check working inconsistently on iOS8

In iOS 8 on both my iPhone 6 and iPad mini 2, most misspelled words are underlined, but I find many misspelled words are not underlined. If I recognize the misspelling and tap the word it will recommend the correct spelling, but the word is not underlined in red. This happens across most apps, including Mail, Evernote, Simple Note, Pages, etc. This causes issues when I'm typing quickly and do not have time to proofread every word.

I can't find a pattern causing the issue. Many common English words are not underlined and it is not limited to one or a few apps.

Any suggestions or recommendations?