Deb 10 -> 11 upgrade and rules no longer load

I had a fully working Prometheus instance running on Debian 10, although I cannot remember what version it was (but it was up to date). Rules were working fine.

I have since upgraded Debian to 11, and Prom is now running version 2.24.1. However my rules files are no longer loading.

The relevant config in the (unchanged) Prom yml is:

# Load rules once and periodically evaluate them according to the global 'evaluation_interval'.
  - /home/prometheus-user/prometheus/rules/*.yml

Two files exist in there, and promtool is successfully able to check and load them:

$ ll /home/prometheus-user/prometheus/rules/*.yml
-rw-r--r-- 1 prometheus-user prometheus 2725 Jan 23 22:03 /home/prometheus-user/prometheus/rules/alerts.yml
-rw-r--r-- 1 prometheus-user prometheus  373 Aug 25  2020 /home/prometheus-user/prometheus/rules/recording-rules.yml

Under Rules in the web ui I see:

No rules defined 

But the configuration seems to have loaded fine via the web ui.

Running Prom with the debug log level doesn’t seem to show anything interesting.

Any help or tips appreciated!

Ok fixed - turns out the new systemd unit uses a ProtectHome=true directive that makes home directories appear empty. Moving the rules to /etc/prometheus had them loading again.