Interesting. I also did not find specific information on SP session timeout...
found only the "system timeout" command on the cluster shell.
I did test it on a 4-node cluster, and the SP session timeout on the nodes are respecting the value on the above mentioned command.
I would open a case to understand what is happening... on 7-mode systems there was an option for the SP session timeout... did not find it for ONTAP.
Regards,
Pedro