I just managed to reproduce a bug where when Symbols are detached unexpectedly.
Steps to reproduce:
Create a symbol "S1"
Create a symbol "S2" that contains "S1"
Copy-paste S1 within S2: It works as expected
Copy S1 from within S2, but paste outside of S1: S1 is detached and turned into a group for no apparent reason.
Drag-and-dropping S1 in the Layers panel outside of S2 also detaches the symbol and turns it into a group.
Expected behaviour: S1 remains a symbol, and is not detached by copy-pasting or dragging it in the layers panel.
Another one that's even simpler to reproduce:
Create a symbol "S1", and another symbol "S2" that contains "S1"
Ungroup or detach S2
S1 was also detached!
Expected behaviour: S1 is still a symbol. I only detached S2.