I often create a set of nested layers and remove auto start symbol timeline actions on symbols within that structure. When I copy the nested elements and past them in a new scene or document the start actions are added back in. As I copied an entire structure with its inherent logic, it makes no sense and adds frustration.
I'd consider adding the initial condition for symbols something that only happens when dragging them to stage from resources or when creating them when converting a structure to a symbol, but not when copying and pasting them, as I want to copy what I have.
I'd go one step further… Setting the starting action by default is absolutely annoying. This should always be a developer 's choice The copy issue comes on top! In most cases I don't even want symbols to be started automatically by a timeline event (since this makes the symbol's animation dependent of what's going on on the main timeline).
Thanks for this report, I definitely agree this doesn't behave quite as expected.
If you do a Paste with Animations it should work a little better, but it does seem like a normal paste probably shouldn't do this. At the very least there's no way to "import with animations" as a workaround in an export/import flow, which makes this much more on the bug side of the fence.
I've also filed a report for having an option so that this is not added, even for new symbols.