this same setup has worked for many years and in all of code 15
zipl version from:
# rpm -q s390-tools
s390-tools-2.34.0-1.1.s390x

topmost changelog entry:
* Mon Aug 26 2024 Nikolay Gueorguiev <nikolay.gueorguiev@suse.com>
* Applied a patch to fix Rust compilation errors (bsc#1229461)
  - s390-tools-2.34-Fix-Rust-compilation-errors.patch
  - Fix: s390-tools: Rust compilation errors (s390_pv_core), build fails with
    "error: unnecessary qualification"
  * Added BuildRequieres in the .spec file
  - perl-Bootloader >= 0.4.15


setup:
/dev/mapper/3600507630bffd2160000000000002000         4120288    347688  
3562888   9% /boot2
s390zl21:~ # multipath -ll
3600507630bffd2160000000000002000 dm-1 IBM,2107900
size=4.0G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
`-+- policy='service-time 0' prio=50 status=active
  |- 0:0:2:1073758240 sdb 8:16  active ready running
  `- 1:0:1:1073758240 sdh 8:112 active ready running

s390zl21:~ # cat /boot2/zipl.conf 
[defaultboot]
defaultmenu = menu

[KIWI_SLES15_20240909]
    image = /boot2/kernel-kiwi-20240909
    target = /boot2/zipl
    ramdisk = /boot2/initrd-kiwi-20240909.xz
    parameters = "hvc_iucv=8 TERM=dumb switch_amode console=sclp_line0
console=ttyS0 bond_mode=1 kiwiservertype=http
kiwiimage=workers/test-s390x-kvm/image-current.xz kiwidebug=1
kiwikernelmodule=scsi_dh_alua ELOG_EXCEPTION=/dev/sclp_line0
ELOG_BOOTSHELL=/dev/ttysclp0
cio_ignore=all,!ipldev,!condev,!f900,!fa00,!fc00,!f100-f123,!f200-f223
crashkernel=64M-:32M net.ifnames=0 sysctl.vm.allocate_pgste=1
transparent_hugepage=never"
...
:menu
    default = 1
    prompt = 1
    target = /boot2/zipl
    timeout = 10

    1 = KIWI_SLES15_20240909


dropping multipath it works again, but it really should keep working
with the device on a multipath map.
s390zl21:~ # umount /boot2
s390zl21:~ # multipathd -k
multipathd> del map 3600507630bffd2160000000000002000 
ok
s390zl21:~ # mount /dev/sdb /boot2
s390zl21:~ # zipl -c /boot2/zipl.conf 
Using config file '/boot2/zipl.conf' (from command line)
Building bootmap in '/boot2/zipl'
Building menu 'menu'
Adding #1: IPL section 'KIWI_SLES15_20240909' (default)
Adding #2: IPL section 'KIWI_SLES15_20240903'
Adding #3: IPL section 'KIWI_SLES15_20240808'
Preparing boot device for LD-IPL: sdb.
Done.
s390zl21:~ # umount /boot2