No "notify" category debug log entries anymore with Bind 9.11.3

[Hildegard Meier]

On old server with Ubuntu 12 and Bind 9.8.1
I got the following "notify" category log entries as expected (domain names and IP addresses redacted):

27-Dec-2021 12:58:51.786 notify: debug 3: zone example.com/IN: sending notify to 1.2.3.4#53
27-Dec-2021 12:58:51.816 notify: debug 3: zone example.com/IN: notify response from 1.2.3.4#53: NOERROR

Log config in /etc/bind/named.conf.options:

logging {

    channel transfer_logfile {
        file "/var/log/named/transfer.log" versions 7 size 10m;
        severity debug 6;
        print-time yes;
        print-category yes;
        print-severity yes;
    };

    category notify         { transfer_logfile; };
};

With new server Ubuntu 18 and Bind 9.11.3
there are no "notify" category debug log entries anymore. The log does only contain entries of severity "info", e.g.

29-Dec-2021 12:00:33.475 notify: info: zone example.com/IN: sending notifies (serial 2021021001)

Log config in /etc/bind/named.conf.options with Bind 9.11.3 is exactly the same as on the old server with Bind 9.8.1:

logging {

    channel transfer_logfile {
        file "/var/log/named/transfer.log" versions 7 size 10m;
        severity debug 6;
        print-time yes;
        print-category yes;
        print-severity yes;
    };

    category notify         { transfer_logfile; };
};

I tested with "severity debug 16" but that did not help.
I tested with "severity notice", that lead to the "info" log entries to disappear, so there is just nothing logged anymore for "notify" category, as expected.

I also run "rndc trace" six times, so in named.log was logged "general: info: debug level is now 6" and did a zone reload, but again, no "notify" debug log entries.

I also tested with "severity debug 6" in the default log channel, that lead to debug log entries for categories like "database" etc., so debug log is generally working.

How can I get the log entries "sending notify to ..." and "notify response from ..." back again in the "notify" category log?

Thanks and Best regards,
Hildegard