It's not so much an issue with every necessarily, perhaps more that we haven't defined the behaviour of completion when there are no _isAvailable = true children, which should probably become its own return early statement.

Are we saying that an "empty" parent should not automatically complete? Whereas an empty parent does currently complete.

Something like:

 checkCompletionStatusFor(completionAttribute = '_isComplete') { 
   let completed = false; 
   const children = this.getAvailableChildModels(); 
   const hasNoAvailableChildren = (!children.length);
   if (hasNoAvailableChildren) return Adapt.checkedCompletion();
   const requireCompletionOf = this.get('_requireCompletionOf'); 

Yes, the problem can be seen when using a plugin which modifies the available children, like a role selector. If a user switches roles, the models which previously had all its children set to _isAvailable: false will be marked as completed.

Preventing completion on models without available children solves this issue, but it can cause a potential problem with trickle where the trickle button is added to an optional last block, which would have previously not been added as the block would be set to completed. Setting "_showEndOfPage": false will resolve this, but requires either a change to the default value, or for the JSON config to be amended accordingly for affected courses.