Two cores disabled.

If you “cat /proc/cmdline” you will see one of the arguments is “isolcpus=1-2”. This is the cause.

In the case of kernel command lines the last argument (if there are copies of the argument) is the one which is active. So instead of removing the “isolcpus=1-2” in the “/boot/extlinux/exltinux.conf” file, simply remove the argument passed to isolcpus, and this is the same as removing all instances of isolcpus (the extlinux.conf APPEND is the last place where arguments are passed to the kernel). Therefore the APPEND key/value isolcpus=1-2 would simply become:
(no “1-2”)