Not a munin user but I am worried if this change catches other users by surprise or if it is not relevant to other users.
Do you see value in having an environment variable to toggle this new behavior? Then based on the environment variable update_seentags() could just write the state content (filtered) to the seentags file and the old behavior is also kept by default.

Old behavior has two fatal errors:

1. Munin reports error if you restart unbound (tag vanishes).
2. New tags will not be included to Munin's graph if they are not present at the first run of munin-async. So basically graph stays empty if you reboot and are using munin-async.

New behavior might include some tags that stays zero forever, but that should not matter. So no, I don't see any point including config option for switching between old/new.

I'll reply inline for others.

LGTM now, thanks for explaining the reason behind the changes.