Having done a bit of testing, this definitely seems to be a regression. I've launched 63 instances of the 10.3 AMI in us-east-1 (ami-031d1abcdcbbfbd8f) and 63 of the current 10.4 AMI (ami-0bb15d03913335eae) on a variety of 6g arm64 instance types. Exacly zero of the 10.3 launches ended up in degraded state because of systemd-growfs failures, while 37/63 (59%) had systemd-growfs fail.
There was a systemd update in 10.4, to version 241-7~deb10u4, which may be related.
