Node 'disappears' when og group checkboxes are left blank in node edit form

I'm on 4.7.4, using og.module 1.110.2.148.

I have several private og groups set up. I set the audience checkboxes to display, with audience required, with any
of the 4 visibility options.

When a user attempts to insert a node without checking any of the private groups, the form prompts -- correctly
-- with an error. That is, user has to check at least one private group.

However, when the user goes back to edit the node, he can uncheck all the private og groups and the node
"disappears". It doesn't even show as a public node, and is only visible to sysadmin.

I check the $node obj and og_public is 0.

I'm looking for a way to either:
1) Ensure the checkboxes canot be nulled during node edit, i.e., at least one must be selected
2) Ensure that node defaults to public -- og_public =1, if all private og groups remain unchecked

Any feedback, hacks, pointers would be much appreciated. Not sure if this is a bug, so haven't reported it as
an issue yet.

Thanks. Mike.