It also can be reproduced on P8 + RHEL7.4,thanks.

The limit is not 32 anymore (except for TCG), the limit is given by KVM:

hw/ppc/spapr.c:
2091 static void ppc_spapr_init(MachineState *machine)
...
2214     if (machine->ram_size < machine->maxram_size) {
2215         ram_addr_t hotplug_mem_size = machine->maxram_size - machine->ram_size;
2216         /*
2217          * Limit the number of hotpluggable memory slots to half the number
2218          * slots that KVM supports, leaving the other half for PCI and other
2219          * devices. However ensure that number of slots doesn't drop below 32.
2220          */
2221         int max_memslots = kvm_enabled() ? kvm_get_max_memslots() / 2 :
2222                            SPAPR_MAX_RAM_SLOTS;

include/hw/ppc/spapr.h:

    649 /*
    650  * This defines the maximum number of DIMM slots we can have for sPAPR
    651  * guest. This is not defined by sPAPR but we are defining it to 32 slots
    652  * based on default number of slots provided by PowerPC kernel.
    653  */
    654 #define SPAPR_MAX_RAM_SLOTS     32

From kernel:

arch/powerpc/kvm/powerpc.c:
    486 int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
    487 {
...
    606         case KVM_CAP_NR_MEMSLOTS:
    607                 r = KVM_USER_MEM_SLOTS;
    608                 break;

arch/powerpc/include/asm/kvm_host.h:

     38 
     39 #define KVM_MAX_VCPUS           NR_CPUS
     40 #define KVM_MAX_VCORES          NR_CPUS
     41 #define KVM_USER_MEM_SLOTS      512
     42 

So the limit is KVM_USER_MEM_SLOTS / 2 = 256

Right, we basically allow more than we used to.

Is the 32 slot limit documented somewhere?  In that case possibly we should update the documentation.