Layer 4 distribution adds the Source and/or Destination ports to the distribution equation (based on the configured distribution mode).
When fragmentation occurs, the first fragmented packet contains the entire 5 tuple. However, the following fragmented packet does not hold the ports.
In this case, each of the following fragmented packets is distributed based on Layer 3 alone. This may cause the fragmented packet to arrive to a different member, making it impossible to assemble all the fragmentation data.