I've just found this too. Running as root from directory /root which is 700 by default I get this:
mymachine~# apt-get download base-files Get:1 http://deb.debian.org/debian buster/main amd64 base-files amd64 10.1 [69.4 kB] Fetched 69.4 kB in 0s (0 B/s) W: Download is performed unsandboxed as root as file '/root/base-files_10.1_amd64.deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied) I understand that apt-get is trying to do a sandboxed download and then switches to unsandboxed when it realizes it can't do a sandboxed download. The main problem I see here is that the message is too verbose and too scary for something which is usually harmless. I would prefer to see something less verbose, for example: W: Not enough privileges to perform sandboxed download (Just an idea) Thanks.