It’s always been this way since the beginning. Pressing “Park” is not required for the despawn to happen. It happens any time a Zoi exits their vehicle, unfortunately.
I made a wishlist for this to change months ago, but the devs expressed concerns about allowing the cars to stay after parking them anywhere because of how it may affect certain parts of the simulation. My hope is that they will be able to make this possible one day.
I believe they’re working on a compromise, but we don’t know what it will look like yet. In the meantime, see my Wishlist entry, if you’d like: No Auto-Disappearing Cars (Let them Park Anywhere)
Side note: If you want to be able to do the extra car features, such as washing the car, repairing it, opening the trunk, etc., you can still do that by clicking the “Get in Vehicle” button as you normally would, then cancelling the interaction before they actually start to get in or on the vehicle. The car will stay where it is without despawning, and if you click on the car again, the extra interactions will be available.