The issue comes from the fact that you have selected “en-GB” as locale. It seems that the British locale expects “Sept” while the American locale (“en-US”) expects “Sep”.
To fix the issue, I think it depends where the data comes from that you are processing. If it was it created using the American locale, changing the locale setting in the node to “en-US” should fix it.
If it was created using the British locale, KNIME (i.e. Java) might treat “September” differently than the tool that created the dates - which would be unfortunate. Instead of changing the locale node setting (as this might have additional implications), you could also use a String Manipulation node and replace “Sep” with “Sept”.