Status from old attempts to solve same nature issues:

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Older (2018) merge request from @raharper:

https://github.com/koverstreet/bcache-tools/pull/1

addressing the fact that kernel uevents would not always emit 
CACHED_UUID parameters, making udev to delete (whenever that happens) 
/dev/bcache/{by-uuid,by-label} symlinks.

This last MR pointed to previous related bugs:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=890446
https://bugs.launchpad.net/curtin/+bug/1728742

And to an upstream kernel patch:

https://lore.kernel.org/patchwork/patch/921298/

to 

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1729145

that wasn't accepted upstream.

Even not being accepted upstream, the SRU was attempted:

LP: #1729145

https://lists.ubuntu.com/archives/kernel-team/2017-December/088680.html
https://lists.ubuntu.com/archives/kernel-team/2017-December/088679.html

Both were NACKED.

Attempted again:

https://lists.ubuntu.com/archives/kernel-team/2017-December/088682.html
https://lists.ubuntu.com/archives/kernel-team/2017-December/088683.html

NACKED again.

And a v2 was sent:

https://lists.ubuntu.com/archives/kernel-team/2017-December/088751.html
https://lists.ubuntu.com/archives/kernel-team/2017-December/088750.html
https://lists.ubuntu.com/archives/kernel-team/2017-December/088749.html

and acked in January 2018 by Coling:

https://lists.ubuntu.com/archives/kernel-team/2018-January/089492.html

but not upstreamed.

BIONIC contains the fix:

commit ed9333e1b583
Author: Ryan Harper <email address hidden>
Date:   Mon Dec 11 12:12:01 2017

    UBUNTU: SAUCE: (no-up) bcache: decouple emitting a cached_dev CHANGE uevent
    
    BugLink: http://bugs.launchpad.net/bugs/1729145
    
    - decouple emitting a cached_dev CHANGE uevent which includes dev.uuid
      and dev.label from bch_cached_dev_run() which only happens when a
      bcacheX device is bound to the actual backing block device (bcache0 -> vdb)
    
    - update bch_cached_dev_run() to invoke bch_cached_dev_emit_change() as
      needed; no functional code path changes here
    
    - Modify register_bcache to detect a re-registering of a bcache
      cached_dev, and in that case call bcache_cached_dev_emit_change() to
    
    Signed-off-by: Ryan Harper <email address hidden>
    Signed-off-by: Joseph Salisbury <email address hidden>
    Acked-by: Colin Ian King <email address hidden>
    Acked-by: Stefan Bader <email address hidden>
    Signed-off-by: Khalid Elmously <email address hidden>
    [ saf: fix incorrect indentation ]
    Signed-off-by: Seth Forshee <email address hidden>

FOCAL contains the fix:

commit 67553dcd7905
Author: Ryan Harper <email address hidden>
Date:   Mon Dec 11 12:12:01 2017

    UBUNTU: SAUCE: (no-up) bcache: decouple emitting a cached_dev CHANGE uevent

GROOVY contains the fix:

commit 67553dcd7905
Author: Ryan Harper <email address hidden>
Date:   Mon Dec 11 12:12:01 2017

    UBUNTU: SAUCE: (no-up) bcache: decouple emitting a cached_dev CHANGE uevent

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So, the kernel patch wasn't accepted, nor bcache-tools patch by 
@raharper, the bcache-export-cached.

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New Upstream summary from @raharper:

https://github.com/systemd/systemd/pull/16317#issuecomment-655647313

in the upstream merge request made by @rbalint.