When using a Change Data Value flow step for a datetime field type and manually entering in a date and time, the time is displayed, but removed from flow step upon refresh.
The reason that the time is removed is because the display field in the flow step is a calendar picker (the drop-down). Calendar pickers do not support displaying the time. It is meant to select a date on the calendar and display that date. It appears to store the value to the record when manually entered in the field, but it will not remain upon campaign refresh.