Seems I found the reason for this behavior.
The trunk is first available on the guests, when you create a VLAN on that trunk and assign this VLAN to the guest. But why was it already there for one out of the three LBs? Probably I was assigning one VLAN for guest2 by mistake to guest1, which triggered to have the trunk available/visible on guest1. After removing the VLAN again from guest1 the trunk is still available. I double checked this with the second LB from this cluster and it behaves exactly as described above.
Maybe this might be a helpful information for someone else out there.
Ciao Stefan :)