the code of the Create Volume command sets the junction path to $null (so it is not mounted into the namespace) for all volumes that are not of type rw. This is probably based on the assumption that you don't want to to configure access on DR mirrors. However, it is a best practice to pre-mount DR volumes so this should probably be changed in WFA, - I'll try to bring this up with the right people.
There are two possible solutions:
Modify the Create Volume command. Note that this will make the command a non-certified command and you won't benefit from future improvements that are made to this command.
Add the Mount Volume command as an additional step to your workflow
Just curious. How big were your volumes? Will your workflow start failing more often if the volume is big and initialization takes longer? Something to consider and perhaps add "Wait for snapMirror" command in the middle.