Comment 9 for bug 1863145

If that is the issue that Stephan is seeing then I have not seen the issue with the bionic package in Dave's PPA:

$ lxc launch ubuntu:08e14ca27965 b
$ lxc shell b
 $ apt-add-repository ppa:waveform/flash-kernel
 $ apt install -y flash-kernel

Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be upgraded:
  flash-kernel
1 upgraded, 0 newly installed, 0 to remove and 182 not upgraded.
Need to get 62.0 kB of archives.
After this operation, 11.3 kB of additional disk space will be used.
Get:1 http://ppa.launchpad.net/waveform/flash-kernel/ubuntu bionic/main arm64 flash-kernel arm64 3.98ubuntu11~18.04.2~ppa1 [62.0 kB]
Fetched 62.0 kB in 1s (84.8 kB/s)
Preconfiguring packages ...
(Reading database ... 28535 files and directories currently installed.)
Preparing to unpack .../flash-kernel_3.98ubuntu11~18.04.2~ppa1_arm64.deb ...
Unpacking flash-kernel (3.98ubuntu11~18.04.2~ppa1) over (3.90ubuntu3.18.04.2) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Setting up flash-kernel (3.98ubuntu11~18.04.2~ppa1) ...
Installing new version of config file /etc/flash-kernel/bootscript/bootscr.uboot-generic ...
Installing new version of config file /etc/initramfs/post-update.d/flash-kernel ...
Installing new version of config file /etc/kernel/postinst.d/zz-flash-kernel ...
Installing new version of config file /etc/kernel/postrm.d/zz-flash-kernel ...