issues with isolcpus

I cannot get the isolcpus command line to take effect.  I have set the grub to the following:
savedefault
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
search --no-floppy --fs-uuid --set=root a08bfa77-56e1-4ee2-a80a-ccc22b8725cd
echo    'Loading Linux 6.3.0-rt11-linuxcnc ...'
linux    /boot/vmlinuz-6.3.0-rt11-linuxcnc root=UUID=a08bfa77-56e1-4ee2-a80a-ccc22b8725cd ro initrd=/install/gtk/initrd.gz 
echo    'Loading initial ramdisk ...'
initrd    /boot/initrd.img-6.3.0-rt11-linuxcnc
GRUB_CMDLINE_LINUX_DEFAULT="isocpus=3"

And have changed the last line to 0,1,2 to 0, to 1, and to 2 with no effect.  The histograms show 4 cores no_isolcpus.
This is an intel i5-6500t.
Any ideas?