Hi Jan,
to reproduce I started a KVM guest with two extra disks:

$ sudo qemu-img create -f qcow2 /var/lib/libvirt/images/test-storage-pools-1.img 30M
$ sudo qemu-img create -f qcow2 /var/lib/libvirt/images/test-storage-pools-2.img 30M
Add to the guest like:
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/test-storage-pools-1.img'/>
      <backingStore/>
      <target dev='vdc' bus='virtio'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/test-storage-pools-2.img'/>
      <backingStore/>
      <target dev='vdd' bus='virtio'/>
    </disk>

This is fix on PCI slot 7 and 8.
So on by-path I can rely on
lrwxrwxrwx 1 root root   9 Nov  6 12:02 virtio-pci-0000:00:07.0 -> ../../vdc
lrwxrwxrwx 1 root root   9 Nov  6 12:02 virtio-pci-0000:00:08.0 -> ../../vdd

Since https://libvirt.org/storage.html#StorageBackendLogical doesn't need host and dir path I didn't define those.
I used:
$ virsh pool-define-as --print-xml testpool1 logical  --source-dev virtio-pci-0000:00:07.0 --target /dev/testpoolVG
<pool type='logical'>
  <name>testpool1</name>
  <source>
    <device path='virtio-pci-0000:00:07.0'/>
  </source>
  <target>
    <path>/dev/testpoolVG</path>
  </target>
</pool>

$ sudo virsh pool-define-as testpool1 logical  --source-dev virtio-pci-0000:00:07.0 --target /dev/testpoolVG
$ sudo virsh pool-define-as testpool2 logical  --source-dev /dev/vdd --target /dev/testpoolVG2

Already on pool-build I see it missing the by path device
$ sudo virsh pool-build testpool1
error: Failed to build pool testpool1
error: Storage pool probe failed: Failed to create filesystem probe for device virtio-pci-0000:00:07.0
$ sudo virsh pool-build testpool2
Pool testpool2 built


The same is true if I pool-build one defined with the syntax you used:
$ sudo virsh pool-define-as testpool2 logical - - virtio-pci-0000:00:08.0 /dev/testpoolVG2


Related log message of the daemon:
 libvirtd[6311]: 6315: error : virStorageBackendBLKIDFindEmpty:3130 : Storage pool probe failed: Failed to create filesystem probe for device virtio-pci-0000:00:07.0

If we enable debug we see this before:
  debug: virStorageBackendBLKIDFindEmpty:3125 : Probe for existing filesystem/partition format LVM2_member on device virtio-pci-0000:00:07.0

In the case of the working /dev/vdd it uses a full path in this call:
  debug: virStorageBackendBLKIDFindEmpty:3125 : Probe for existing filesystem/partition format LVM2_member on device /dev/vdd

Now it is not the code of libvirt on pool-build that "looses" that information.
No if we look more carefully at the XML that was defined it is:
  <device path='virtio-pci-0000:00:07.0'/>
Silly me :-/ I forgot the full path in the definition 
Redefine as:
$ sudo virsh pool-define-as testpool1 logical  --source-dev /dev/disk/by-path/virtio-pci-0000:00:07.0 --target /dev/testpoolVG

Yeah that looks better:
$ sudo virsh pool-dumpxml testpool1
<pool type='logical'>
  <name>testpool1</name>
  <uuid>d85ec218-9b82-4f99-8b75-6b11840d06b5</uuid>
  <capacity unit='bytes'>0</capacity>
  <allocation unit='bytes'>0</allocation>
  <available unit='bytes'>0</available>
  <source>
    <device path='/dev/disk/by-path/virtio-pci-0000:00:07.0'/>
    <name>testpool1</name>
    <format type='lvm2'/>
  </source>
  <target>
    <path>/dev/testpool1</path>
  </target>
</pool>

$ sudo virsh pool-build testpool1
Pool testpool1 built

Ok now I can build the by-path pool as well (as you did without the silly mistake in between).

$ sudo virsh pool-start testpool1
error: Failed to start pool testpool1
error: unsupported configuration: cannot find any matching source devices for logical volume group 'testpool1'

$ sudo virsh pool-start testpool2
Pool testpool2 started

So far - confirming your report