The use case is that we have added additional licensing parameters to the LMA License object that our managed package reads from the LMA via a @RestResource site added to the LMA org.
This works fine for production orgs that have an LMA License object but not for sandboxes that do not have a corresponding LMA License object. Hence the quick fix would be to identify the production org id (when in the sandbox org) and always pass that in the read request...
PS The lack of a license for sandboxes is awkward in other situations too - see LMA should create licenses for sandbox installs.