Default LWM2M lifetime setting does not work

Question

I have the following: - NRF52840 - BG96 modem - NCS 1.5.1 - Zephyr v2.4.99-ncs2 
 When I start the firmware, it will first bootstrap followed by leshan server registration. In my prj.conf I have: CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=900
 
 But when I look at the tcpdump I see lt=20. 
 I also checked with grep: grep -R CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME *
build_nrf52840_orbitz/zephyr/include/generated/autoconf.h:#define CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME 900
Binary file build_nrf52840/zephyr/zephyr_prebuilt.elf matches
build_nrf52840/zephyr/zephyr.lst:	lifetime[index] = CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME;
build_nrf52840/zephyr/zephyr.lst: new_lifetime = CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME;
build_nrf52840/zephyr/zephyr.lst: new_lifetime = CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME;
build_nrf52840/zephyr/.config.old:CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=900
build_nrf52840/zephyr/.config:CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=900
build_nrf52840/zephyr/misc/generated/configs.c:GEN_ABSOLUTE_SYM(CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME, 900);
Binary file build_nrf52840/zephyr/zephyr.elf matches
Binary file build_nrf52840/zephyr/libzephyr.a matches
Binary file build_nrf52840/zephyr/CMakeFiles/zephyr.dir/misc/generated/configs.c.obj matches
prj.conf:CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=900
 
 Somehow this setting is being overruled. There's no mention of overrule in the documentation: https://docs.zephyrproject.org/1.14.0/reference/kconfig/CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME.html 
 Also according to the documentation, the default is 30. 
 Where does 20 come from? and how do I change my project to use lifetime 900?