AFAIK, MESH model 1.0 specification defines each model messages not to be fragmented.
So, when we use SIG-defined model, each message SHOULD NOT be fragmented, if we make a size mistake.
According to the specification, a single packet can deliver 8,9,10 bytes of message data when using 3,2,1 bytes opcode each.
But, when I tested with MS_access_publish() function, packet fragmentation occurs when more than 5,6,7 bytes are in a packet.
In the other word, MESH SDK handles 3 bytes less than specification.
I am strongly suspicious of unpacked structures which is used in message store. I changed them myself, but no effects.
So, I think the mesh library may have the same issues inside.
Please check about this issue.