Comment 4 for bug 1917590

Also /run/casper-md5check.json is missing in the live system, as is /var/log/installer/casper-md5check.json in the installed system.

Since this is a relatively new problem, I did see this commit, which sounds like it attempts to continue to boot while the check is running. I wonder if the problem doesn't lie here somewhere:
https://git.launchpad.net/ubuntu/+source/casper/commit/?id=8415b4102e5c59064a5acff878aa2c3c8aa10e96