I did some more tests, and I can confirm the behavior you are seeing.
When a new VM is created on the datastorecluster it is not automatically added to the VmConfig array as a StorageDrsVmConfigInfo entry.
The fact that you see it in the vSphere client is due to the defaults (see for example the description of the enabled property).
I'm not sure if this is intentional or a "feature".
My guess is that this is intentional, as long as you don't divert from the defaults, there is no need to create an entry.
Perhaps some SDRS wizard can shine his light on this ?