Outlook.com Spell Check

That partial-word underline looks like the spell checker may not be refreshing correctly as you type, rather than an actual spelling mistake.

Try deleting and retyping one affected word, including the space after it. If the underline returns, check whether both your browser’s spell checker and an extension such as Microsoft Editor or Grammarly are enabled. Temporarily turn one off to see whether that makes a difference.

Which browser are you using? Outlook.com can have more than one spelling tool checking the same text.