I'm trying to change the gpu memory on my rp5 8GB.

root@raspberrypi:/boot# cat config.txt  | grep mem
gpu_mem=256
gpu_mem_256=64
gpu_mem_512=128
gpu_mem_1024=256

and after rebooting multiple times I never get the memory to change :

root@raspberrypi:/boot# vcgencmd get_mem arm && vcgencmd get_mem gpu
arm=1020M
gpu=4M