Calendar start location isn’t always available

Sometimes when I set an event in Calendar to an address, the software offers me my start location and automatically calculates travel time to my destination. This is helpful because it sets the alert to that much time plus what I select before departure. Sometimes I only get offered fixed travel times, like 15, 30 minutes, or 1 hour to choose. This happens even though the Maps app routes me and gives me a fixed travel time to the same address. Is there something I can set or do to always get the automatic travel time from my start location?