Automatic subdomain wildcard for DHCP-DDNS additions

Is your Linux DHCP client asking for dynamic DNS update? I can't see any information in your question about that, and without it your Linux DHCP requests won't get DDNS registration on the server side. Look at the "DYNAMIC DNS" section in dhclient.conf(5) for details of the settings required.

The ddns-domainname and ddns-rev-domainname options are not necessary, since your style is interim (as it should be).

ignore client-updates is supposed to ignore whatever it is that the client thinks their FQDN should be. You're doing that correctly.

It is odd that you are not specifying option domain-name "example.net" where example.net is your domain name. This can be global or it can be scope-specific (you've omitted your scope for some reason; it might have been important).

However, I see that the problem is it thinks the DNS suffix of your second server is my.domain.net.my.domain.net.. It thinks this because, although it is correctly ignoring the server's assertion as to its DNS suffix (or domainname), server2 is telling the DHCP server that its hostname (to which the FQDN suffix would be added) is server2.my.domain.net. The hostname shouldn't have any .s in it. While it is odd and perhaps a misfeature that dhcpd does not strip this or reject it, server2 is misconfigured and that is the only place that you can correct this.

Before the interim method was established, for static leases, it was possible to use the ddns-hostname option to override the client's assertion. This is no longer usable.

Even now, for clients which accept their hostname from the DHCP server as specified in a static lease (most linux and several other things, but not windows), you can control this behaviour from the DHCP server. However, it doesn't sound like this describes your environment.