> What release is it being used for rescue mode? 

Looks like bionic

I went to reproduce this last night and I deployed two systems again with Bionic, selecting the GA kernel. Once I logged into the deployed systems I got the following kernels:

ubuntu@nexus:~$ uname -a
Linux nexus 4.15.0-36-lowlatency #39-Ubuntu SMP PREEMPT Tue Sep 25 00:16:08 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
ubuntu@falcon:~$ uname -a
Linux falcon 4.15.0-36-lowlatency #39-Ubuntu SMP PREEMPT Tue Sep 25 00:16:08 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

I did not select lowlatency, so I am confused why that was used.

When I booted into rescue mode I saw the following kernel:
ubuntu@nexus:~$ uname -a
Linux nexus 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
ubuntu@falcon:~$ uname -a
Linux falcon 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux


Logs and package versions below:

maas.log
http://paste.ubuntu.com/p/hNjQtVcqCr/

rackd.log
http://paste.ubuntu.com/p/fthSWGhk9s/

regiond.log
http://paste.ubuntu.com/p/yvyqkh5fpp/

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                            Version                        Architecture Description
+++-===============================-==============================-============-=================================================
ii  maas                            2.4.2-7034-g2f5deb8b8-0ubuntu1 all          "Metal as a Service" is a physical cloud and IPAM
ii  maas-cli                        2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS client and command-line interface
un  maas-cluster-controller         <none>                         <none>       (no description available)
ii  maas-common                     2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS server common files
ii  maas-dhcp                       2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS DHCP server
un  maas-dns                        <none>                         <none>       (no description available)
ii  maas-proxy                      2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS Caching Proxy
ii  maas-rack-controller            2.4.2-7034-g2f5deb8b8-0ubuntu1 all          Rack Controller for MAAS
ii  maas-region-api                 2.4.2-7034-g2f5deb8b8-0ubuntu1 all          Region controller API service for MAAS
ii  maas-region-controller          2.4.2-7034-g2f5deb8b8-0ubuntu1 all          Region Controller for MAAS
un  maas-region-controller-min      <none>                         <none>       (no description available)
un  python-django-maas              <none>                         <none>       (no description available)
un  python-maas-client              <none>                         <none>       (no description available)
un  python-maas-provisioningserver  <none>                         <none>       (no description available)
ii  python3-django-maas             2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS server Django web framework (Python 3)
ii  python3-maas-client             2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS python API client (Python 3)
ii  python3-maas-provisioningserver 2.4.2-7034-g2f5deb8b8-0ubuntu1 all          MAAS server provisioning libraries (Python 3)