Comment 1 for bug 1803749

This is caused by unattended-upgrades having an ActionGroup during the clear. So, UnattendedUpgrades cache should likely overwrite clear() and make it reset the action group, for example (after making actiongroup global):

 def clear(self):
   global actiongroup
    actiongroup.release() # pyflakes
    apt.Cache.clear(self)
    actiongroup = apt_pkg.ActionGroup(cache._depcache)