Okay…now this is starting to look like a stupid user error – I have completely missed the change in how the backup management is currently working. So Discourse now manages and deletes S3 backups directly, without the need to purge the old backups with a bucket rule? Now increasing the value to 30, as backups should not eat up local disk space.
The number of backups that were stored in the S3 buckets did not match the setting 3 though.