Due to the config file purging, removing the last instance of a php fpm pool for a given version of php, which does not have that version required elsewhere, removes all of the php versions config. This causes the php version to no longer be configured, and the fpm service for that version to fail on start.
The behaviour is partly correct - the version of php should be removed if it's no longer required; however this should be done cleanly by shutting down the service and removing the php install.
Due to the config file purging, removing the last instance of a php fpm pool for a given version of php, which does not have that version required elsewhere, removes all of the php versions config. This causes the php version to no longer be configured, and the fpm service for that version to fail on start.
The behaviour is partly correct - the version of php should be removed if it's no longer required; however this should be done cleanly by shutting down the service and removing the php install.