Hi,
This is a super frustrating bug.
I have an ad-hoc subprocess from which it is possible to call other subprocesses
Each of these subprocesses has it own diagram.
Here is the really frustrating part.
When I connect the subprocess to the process, the process disappears from the model explorer.
This makes absolutely no sense 
Why would VP delist the processes that are being called as subprocesses?!?!?!
Please advise.
Could you describe a little bit more about “connect the subprocess to the process” ?
e.g. actual steps you performed
Hi Roy
Please see below what I did to “connect” (linking) a subprocess to an existing process diagram.
After this, if I try to add the linked process to the model explorer, the link in the diagram is broken.
If I try to reestablish the link in the diagram, the process stops being listed in the model explorer.
Makes better sense now?
Thank you,
Filipe
When link Business Process Diagram to Sub Process,
Sub Process will become parent of the Business Process Diagram.
As a result, those shape inside the Business Process Diagram will become child of Sub Process.
When another Model add Business Process Diagram as sub diagram.
It means Business Process Diagram’s parent changed to the Model.
It will automatic break the “linking” because each Diagram should only have one parent.
If you want to organize diagram in folder(s) without breaking its own structure
I suggest using Logical View
You can show Logical View by follow below screenshot.
In Logical View, you can create folder (View) and add existing diagram you want.
(even can add same diagram into more that one folders)
As a result, diagrams are organized and the linking remain unchanged.
Hope it help.
Reference to BPMN 2.0 spec. Section 13.2.4
Sub-Process/Call Activity
A Sub-Process is an Activity that encapsulates a Process
Seems it means a SubProcess will own the process. It is not used to ‘link’ to another process.
So, should you use CallActivity to link to your existing process?
PS: I am not familiar with BPMN. Sorry if I got wrong on the definition of Process & SubProcess. 
