I think these locale errors are harmless. If you want to avoid them, you can run sudo dpkg-reconfigure locales and select the locale(s) you want to generate.
I had the same problem and ran the following command:
$ sudo apt remove fuse && sudo apt install fuse3 && sudo apt upgrade
Although I got the same warnings about locales, everything went smoothly, so I ran unattended-upgrade:
$ sudo unattended-upgrade -d
As a result, libntfs-3g883 was removed, and several packages are kept back.
Package at-spi2-core has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package batctl has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-bridge has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-networkmanager has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-packagekit has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-pcp has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-storaged has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-system has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package cockpit-ws has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package freedombox-doc-en has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package freedombox-doc-es has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package guile-2.2-libs has a higher version available, checking if it is from an allowed origin and is not pinned down.
Package guile-2.2-libs is kept back because a related package is kept back or due to local apt_preferences(5).
Package libatspi2.0-0 has a higher version available, checking if it is from an allowed origin and is not pinned down.
Extracting content from /var/log/unattended-upgrades/unattended-upgrades-dpkg.log since 2021-09-02 14:21:30
Nevertheless, it appears I am now running v. 21.7. Should there be any cause for concern?
My Pioneer FreedomBox is not upgrading beyond 21.4.4~bpo10+1 though it managed to dist-upgrade to Bullseye successfully.
The following packages have been kept back:
freedombox guile-2.2-libs sshfs zile
I wonder if this is also related to fuse3.
For freedombox, guile-2.2-libs and sshfs, probably. For zile I don’t know, I don’t have it. You can check with “apt -s install packageX” for each upgradable package.
Regarding packages kept back after upgrade to bullseye, see issue #2103
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@lifeform thanks! Is it recommended then to run sudo apt full-upgrade? Also, no where does it mention libntfs-3g883 being removed as it was on my installation 
I am hesitant to upgrade from the command line unless necessary. In the past I have had prompts regarding configuration files and I did not know how to answer, resulting in a broken installation.