"last_time_reachable" attribute within device_tracker Fritzbox integration

Hi folks,
I was looking a long time for a solution to see how long a device wasn´t online.
I recognized the “last_time_reachable” attribute for my fritzbox device_tracker integration that was implemented with the june release (?).

Works great for me, as long I´m restarting HA. After that, the attribute is disappearing from all devices which are not_home.

Is this by design or can I somehow keep the values after a restart?

Before restart:

source_type: router
ip: 192.168.1.128
mac: 60:B7:6E:01:67:F2
host_name: android-pixel5
last_time_reachable: '2021-06-07T17:24:43+00:00'
friendly_name: Pixel 5
icon: mdi:lan-connect

after restart, the attribute is completly missing when the device is not_home:

source_type: router
ip: 192.168.1.128
mac: 60:B7:6E:01:67:F2
host_name: android-pixel5
friendly_name: Pixel 5
icon: mdi:lan-connect