That probably means you have your permissions/sleep settings on your phone setup in a non-standard way. None of my phones work this way. Whenever I'm working in the garage, or working on another car, my Tesla never locks. I can't remember the exact wording, but after a while I'll even get a notifications that the doors are still unlocked.. However, if I walk away, it will lock.If I stand next to the car for a few seconds after I get out and close the door it will lock anyway even if I don’t walk away. Done it many times. I just have to time how many seconds it takes and test various scenarios to see if the timing varies.
Speaking as someone that actually worked on proximity detection, and walk away lock/unlock in a previous life, my educated guess on why the walk away lock distance is different between the front and back of the car, has everything to do with wireless signal attenuation. When you work in these things, you'll find that antenna placement and chipset used in your handset plays a big role. Some chipsets will have much worse signal attenuation going thru objects than others. For example, when I was doing testing, I found one particular chipset, had the same signal attenuation going thru one wall, as another chipset did going thru four walls.
Also, there is significant signal attenuation going thru the human body.. So depending on where on your person you place your phone, and which way you are walking away from the car, the signal attenuation will be much worse if the signal from your phone has to go thru your body, etc. This is why for example, you'll get much different results if you put the phone in your back pocket, vs front pocket, vs in your purse, vs buried in your backpack, etc. There are different ways to try to mitigate these things, but they draw significant amounts of power on the phone side. This stems from the fact that BT proximity probes are required to be transmitted at maximum power on both sides, so you want to try to avoid sending too many of these probes.. Likewise, some of the sensors on the phone side that you'll want to try to use to minimize when you send these probes, actually draw a significant amount of power too, which is why the OS will tend to coalesce these requests and space them out, unless you force the OS to query immediately, etc.