Hi, thanks for your reply.

   Are you sure Tor is runnig under user ?pi? and not ?debian-tor?? What 
is the value of the ?User? option in torrc? Who is the process owner: 
see in `htop`, `top` or similar?

Yes. I am starting tor as user pi with: /usr/local/bin/tor --quiet . Therefore the process is owned by "pi", as htop shows.

There is no specific user specified in the torrc.

   Another question is, what?s the reasoning behind switching from a 
dedicated user to the default system account??? shared with other services?

I only did this, because I could. My thought of this was: Run it as "pi" the main thing is that it is not root. 

? No Raspberry Pi OS Buster here, but googling indicates ?pi?
   is the default user on a fresh install.

Yes, it is.

Why did the owner of the log files changed? Did tor this or the OS? It is solved by setting the user to "pi" in the torrc?

regards,
Patrice