Fails to remove /var/lib/command-not-found/ when package is purged

This concerns command-not-found 20.04.2 in Ubuntu focal.

The apt-get invocation largely tells the story:

# apt-get --purge --autoremove remove command-not-found
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  command-not-found*
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 35.8 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 95222 files and directories currently installed.)
Removing command-not-found (20.04.2) ...
(Reading database ... 95216 files and directories currently installed.)
Purging configuration files for command-not-found (20.04.2) ...
dpkg: warning: while removing command-not-found, directory '/var/lib/command-not-found' not empty so not removed

# ls -l /var/lib/command-not-found/
total 3020
-rw-r--r-- 1 root root 3084288 May 8 14:54 commands.db
-rw-r--r-- 1 root root 2442 May 8 14:54 commands.db.metadata

I'm purging the package, these files clearly belong to it, that directory should be gone.

(Side note: apt-get should be reporting "python3-commandnotfound is no longer needed," since command-not-found is the only package that depends on it, but for some reason this isn't happening. That's why I had specified the --autoremove; I was expecting to remove two packages.)