It seems the DateTime to String node has an issue with the milliseconds (or fractions of a second).
Here is an example - the preview seems correct (it shows the milliseconds)
just test it and works fine for me. Which KNIME version are you using?
To add: In this node, preview value is not time from your column but the time when you opened node for the first time. Is it possible you don’t have any milliseconds in your column?