It does seem logical that it should allow updating a template network.
If the call does not fail because the network ID is a template, then it should report fail if it did not update the SSID.
But if it says it completed successfully, that is incorrect because the SSID was not changed, there is a contradiction here.
I had a look at overall documentation, but it doesn't say if a template network is treated the same as a network when making updates.
It won't get you out of our current situation, but you could open a support case to report that it doesn't work.
Either it's a bug or the documentation needs to clarify things, but it should not be necessary to guess!