Mounting new filesystem affects non-recursive bind mounts?

You should be able to see that new mounts stop being copied, if you run mount --make-private on the mount point.

The difference when running bash as init, is that the source filesystem has been mounted as private.[*] Whereas booting the full system is effectively running --make-shared. You can see the difference by looking at findmnt -o +PROPAGATION.

Once the root filesystem is mounted as shared, any filesystem mounted directly under it will inherit the same propagation setting.

The root filesystem is being remounted as shared by systemd. This feature was added to systemd around 2012. It was discussed on the amazing Arch Linux wiki.

https://wiki.archlinux.org/index.php?title=Talk:Systemd&oldid=411350#Systemd_defaults_.2F_to_rshared.2C_gotcha

https://github.com/systemd/systemd/commit/b3ac5f8cb98757416d8660023d6564a7c411f0a0

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While you're reading this, I also recommend learning how to safely disassemble a recursive bind mount. Because on shared mounts, mounts and unmounts propagate in both directions :-).

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[*] Booting with init=/bin/bash, I saw the filesystems mounted as private. Even though I was still booting with Fedora's dracut initramfs, which runs systemd internally. I'm not 100% sure what was happening here.