Rounding this one out, the support team were very helpful getting this sorted.
it turned out to be an issue with policy objects and a sequence of events.
Due to working around a separate (non-meraki) issue, I had previously done the following:
- Deployed the network from template
- Unbound from template (successfully)
- Rebound the network to the template (successfully).
Because of this sequence of events, and the fact that policy objects were in use in an L3 firewall rule, there became an underlying mismatch (for lack of better wording) between these policy objects which prevented the template from being unbound. I ended up removing the policy objects from the rule which then allowed me to successfully unbind.