When I copy a Symbol to the clipboard and then paste it, the resulting paste is no longer a symbol — it's been "expanded". Is this new behavior?
I'm using Illustrator 25.2.3 on macOS 11.3.
That is not normal behaviour.
My system is older and I cannot replicate that.
Do you have clipboard enhancing software running?
Does it happen after a restart with only Illustrator running?
You diagnosed the problem correctly: I am running a clipboard enhancement called "Paste" (https://pasteapp.io) — never would have dreamed it would affect behavior in this way!
Thank you for the suggestion.
Glad to hear you've got it working.
How copy and paste works between Illustrator to InDesign is defined by the preferences, how you set them up.
In InDesign in the Clipboard Section.
In Illustrator in the Clipboard Section.
In older versions you have to look into file handling.
But I would recommend to save as PDF or AI in Illustrator and place in InDesign as linked file.