Yup it only does it at certain times.
From the release notes: "to Work when leaving Home in the morning (Monday through Friday) and Home when leaving Work in the afternoon or evening (Monday through Friday)."
And from the manual: "In addition, if you are Home and get in your vehicle on weekdays (Monday-Friday) from 5AM-11AM, Model 3 will automatically route you to your predetermined Work destination (see Recent, Favorite, Home and Work Destinations on page 125). If you are at work on weekdays from 3PM-11PM, your vehicle will automatically route you Home."
You could probably do automatic navigation with calendar events instead.