OK, I did some more experimenting. It’s definitely Keyscape misbehaving, not Cantabile. All other plugins perform fine. Keyscape even misbehaves when there is only one instance, in one song (and no setlist loaded).
Seems like a pattern (like @brad mentioned) - I can find plenty complaints about keyscape here - even one of mine: https://community.cantabilesoftware.com/t/ridiculous-number-of-file-handles-in-keyscape/4701/2