Thinking of possibilities:
1. Might be created as a cluster diskgroup. (If so, need to switch it back to non-cluster secondary diskgroup if there is no cluster services)
Is the diskgroup a "cluster" type diskgroup? (Did you create the diskgroup with the cluster option checked?)
If is was created as a cluster diskgroup, then it is normal behaviour for it to not auto-import upon reboot.
If it is not a cluster diskgroup but just a standard diskgroup, it should import upon reboot via the VxDGDI service. (delayed import)
2. VxDMP DSM multipathing. May need to install latest DDI package or something isn't configured properly.
Do you have VxDMP DSM DDI package installed? or did you select DMP when you installed SFW?
(You may see DMP DSM listed in the VEA under the Storage Agent and see disks under the VITARGET if so)
If you are using it, there is a possibility that something is setup wrong with it.
3. Issues with iscsi connection / access to target after a reboot.
Once you reboot, you say the disks are still logged in. At this point, in VEA, do you see the diskgroup at all? (in any state)
Or do you just see the iscsi harddisks down the disks list but do not see any diskgroups?
Maybe you could try using the Windows disk management and see if it works after the reboot also or if it's just SFW not working.
Let me know if anything seems to match.