isolcpus is not working as expected on kernel 5.15

uname -a
Linux compute1 5.15.0-79-generic #86-Ubuntu SMP Mon Jul 10 16:07:21 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

We are using linux-generic package and starting up the machine with:

GRUB_CMDLINE_LINUX="isolcpus=4-127"

in /etc/default/grub

We observe some processes run on 64-67 cpus despite isolcpus=4-127 being set properly.

We don't observe this behaviour with kernel 6.2 (linux-hwe) on the same machine above or other systems we have had for a while which use 5.4 kernel (ubuntu 18.04)

Packages we have tested with unexpected behaviour on jammy:
linux-generic
linux-lowlatency
---
ProblemType: Bug
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Aug 16 09:44 seq
 crw-rw---- 1 root audio 116, 33 Aug 16 09:44 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.11-0ubuntu82.4
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
CasperMD5CheckResult: unknown
DistroRelease: Ubuntu 22.04
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb:
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 003: ID 046b:ff10 American Megatrends, Inc. Virtual Keyboard and Mouse
 Bus 001 Device 002: ID 046b:ff01 American Megatrends, Inc. Virtual Hub
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Lsusb-t:
 /: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/10p, 5000M
 /: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/16p, 480M
     |__ Port 8: Dev 2, If 0, Class=Hub, Driver=hub/5p, 480M
         |__ Port 4: Dev 3, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
         |__ Port 4: Dev 3, If 1, Class=Human Interface Device, Driver=usbhid, 1.5M
MachineType: Inspur NF5180M6
Package: linux (not installed)
PciMultimedia:

ProcEnviron:
 LC_CTYPE=UTF-8
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB: 0 astdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.15.0-79-generic root=UUID=9859baba-bd9f-41a3-a259-64a3a97b4584 ro isolcpus=4-127 intel_pstate=disable acpi=force default_hugepagesz=2M hugepagesz=2M hugepages=2048 transparent_hugepages=never
ProcVersionSignature: Ubuntu 5.15.0-79.86-generic 5.15.111
RebootRequiredPkgs: Error: path contained symlinks.
RelatedPackageVersions:
 linux-restricted-modules-5.15.0-79-generic N/A
 linux-backports-modules-5.15.0-79-generic N/A
 linux-firmware 20220329.git681281e4-0ubuntu3.14
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
Tags: jammy uec-images
Uname: Linux 5.15.0-79-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: N/A
_MarkForUpload: True
dmi.bios.date: 08/27/2022
dmi.bios.release: 5.22
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 06.00.05
dmi.board.asset.tag: 28AC19701
dmi.board.name: NF5180M6
dmi.board.vendor: Inspur
dmi.board.version: YZMB-01642-107
dmi.chassis.asset.tag: 28AC19701
dmi.chassis.type: 23
dmi.chassis.vendor: Inspur
dmi.chassis.version: 0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr06.00.05:bd08/27/2022:br5.22:svnInspur:pnNF5180M6:pvr0:rvnInspur:rnNF5180M6:rvrYZMB-01642-107:cvnInspur:ct23:cvr0:skuDefaultstring:
dmi.product.family: Not specified
dmi.product.name: NF5180M6
dmi.product.sku: Default string
dmi.product.version: 0
dmi.sys.vendor: Inspur