I confirmed this is a different problem than on Ubuntu. Or at least the solution for Ubuntu doesn't work on Debian. I hate to leave things broken but I've spent days on this and I have no more left to give.
The writing is on the wall: using bubblewrap this way is a minefield. Since the goal of Dropserver is to be easy to install (eventually) having to troubleshoot this is going to be a pain. I'd rather focus on a blast-radius sandbox at the service level. Besides, when I make releases for Mac and maybe even Windows, there likely won't be the option to do this level of sandboxing? So I think it's best to focus on Deno as the only sandbox for individual appspaces, and a ds-host level sandbox (hardened systemd service for example) to contain any escapes.