To reproduce: add a RadSpellChecker and a RadTextBoxControl and set the RadSpellChecker.AutoSpellCheckControl property to the RadTextBoxControl. When you type "Thisis", the red underline appears, indicating the error. However, if you use the context menu to correct the misspelled word, the red underline remains. Please refer to the attached gif file.

Workaround: stop the default context menu and use the spell checking form instead:

private void radTextBoxControl1_ContextMenuOpening(object sender,TreeBoxContextMenuOpeningEventArgs e)
{
    e.Cancel = true;

    this.radSpellChecker1.Check(this.radTextBoxControl1);
}