This happens because during boot we ran started systemd-remount-fs.service which applied mount options from fstab to root mount and remounted it RW in the process. Later on something failed (different mount) and then systemd isolated emergency.target. However, isolating some target doesn’t undo effects of services that finished before that. Though we might want to make an exception in this specific case. I will ask upstream developers on their opinion.

Two things:

First, context: I am a Red Hat Academy instructor. The RH134 course content explicitly states that / will be mounted RO if /etc/fstab has a non existent device or UUID, nonexistent mount point, or incorrect mount point option.

If mounting / as RW is intended, and not a bug, then course content needs changing - and possibly other documentation as well. I understand that this may not be in the purview of Red Hat Bugzilla group.

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Second: if emergency.target is supposed to mount / as RO (as it does when systemd.unit=emergency.target does when added to the boot loader), shouldn't the systemctl isolate emergency.target command have the same effect?

Just to add to the documentation angle, there is this: "In emergency mode, the system mounts the root file system only for reading, does not attempt to mount any other local file systems..."

From here: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/managing-services-with-systemd_configuring-basic-system-settings

After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.