2021-07-29 08:02:43,693: DEBUG - dpkg: unrecoverable fatal error, aborting:
2021-07-29 08:02:43,693: DEBUG - unknown system user 'matrix-synapse' in statoverride file; the system user got removed
2021-07-29 08:02:43,693: DEBUG - before the override, which is most probably a packaging bug, to recover you
2021-07-29 08:02:43,693: DEBUG - can remove the override manually with dpkg-statoverride
That’s a new one… 
Let’s follow apt’s advice:
dpkg-statoverride --list will give you a list of the overrides, check out which paths rely on matrix-synapse. (weirdly I have none on my server with synapse).
You can then remove them with dpkg-statoverride --remove <path>.