Update – in troubleshooting another issue for another website (I’ve had several today after updating sites to WP 5.5) I found that installing the WordPress-developed plugin “Enable jQuery Migrate Helper” fixes the main issue. The tree now shows and is functional again.
However, the order of my tree is now in reverse order from how I’d like it, and changing the order from “asc” to “desc” makes no difference. My orderby title results in a Z-to-A sort. Is there a way to remedy this?