There is a big problem when you change a widget's nested cells from shared to individual. Steps to reproduce:
Create a new document (doc A)
Create cell (cell A)
Create another cell (cell B), nest it in cell A
Select cell A and create a widget out of it and cell B
Copy the widget from doc A to a new document, doc B (pasted instance of widget is instance A)
Return to document A, select cell A
Change cell A’s nested cells from shared to individual
Copy the widget from doc A to doc B (instance B), updating the widget definition in doc B
Result: cell B in instance A disappears! Not good.
I can reproduce it and will have a look what goes wrong here.
Thank you for the detailed bug report.