I've checked our system and there hasn't been an update on this one, currently the only workaround is for allowing Access to determine the Permissions for POSTs.
I'm not exactly sure but this seems to be a bug in our system and our development team is working to resolve it.
This issue is not only related to guests but it affects the editing permissions of some user roles as well. The only workaround that the support team was able to come up with is for having access to manage the default POST CPT as well to help customer restore some functionality while the issue is investigated and a more permanent fix is provided by our development team.