Hello and welcome to the forum.
Not being a big fan nor frequent user of Cortana, I'm probably not the best person to be advising, but....
All I can say is that when we see problems such as this with Cortana, it is typically a misconfiguration of Region and Language settings.
For instance if Cortana knows based on your location settings that you are in the UK but you have chosen English US as your language / input method it may cause this type of issue.