External forest domains not showing in task sequence options

[Norphus]

I have a big project on at the moment. I'm building a new AD domain forest and a ConfigMgr site hierarchy to manage it. The new forest is built. There is a transitive trust set up between the old forest and the new one to aid in migration. My ConfigMgr hierarchy is also set to publish itself to the old AD forest. This all appears to work; the old forest is in ConfigMgr, ConfigMgr is publishing its sites to the old forest and discovery in the old forest is working quite nicely.

The new forest isn't going into production until August/September but I'd like to start using ConfigMgr now to manage workstations and to help get my teams familiar with it before we start using it in anger in the summer.

The issue that I have is that the domain in the old forest is not appearing in the "Pick a domain" list when I'm creating a OSD task sequence. The domains that do appear are the ones in the new forest:

This admittedly isn't much of an issue, I can just manually type in the name of the old domain, type in the path of the OU that the PC is going into and no problem. It is pretty annoying though and I'd like to know if there's anything obvious I've missed in setting up this cross-forest management.

Thanks,

Ian