As far as I can tell, EventBus just forwards whatever is given as the $user param of the ChangeTagsAfterUpdateTags.

If something has changed, it is likely to be in the MediaWiki hook.

Out of curiosity, and in hoping to make this easier to fix, I grabbed daily aggregates for October 2023. Looks like something was deployed the week of Oct 23 as that's when the proportion increases from the expected 6–7%:

log_date	num_tagged_edits	num_tag_edits_with_null_performer	prop_null
2023-10-23	985895	64943	0.065872
2023-10-24	961813	65499	0.068100
2023-10-25	1133200	275178	0.242833
2023-10-26	1118374	926144	0.828117
2023-10-27	1241023	1241020	0.999998
2023-10-28	1157623	1157620	0.999997
2023-10-29	1151495	1151490	0.999996

Including MW-Interfaces-Team who (I think) support hooks. Let me know if not.

The ChangeTagsAfterUpdateTags hook is fired by ChangeTagsStore:.updateTags():

	public function updateTags( $tagsToAdd, $tagsToRemove, &$rc_id = null,
		&$rev_id = null, &$log_id = null, $params = null, RecentChange $rc = null,
		UserIdentity $user = null
	)

As you can see, the $user parameter is optional, some callers just don't supply it. The documentation for the parameter says: Tagging user, in case the tagging is subsequent to the tagged action. I don't know what that means, exactly, but it's the same in the documentation of the hook itself: the parameter is optional.

Looking at callers, I find that ChangeTagStore::addTags() and ChangeTag::addTags() never supply a user, the $user parameter is just absent. They are used in quite a few places, including ApiTag. I suppose that's where the nulls come from. But I don't see what changed, it seems like these methods never supported passing a user. The last time the method sigature was touched was in 2014.

Maybe it used to fall back to the user from the main RequestContext? But then, why is the $user parameter in the hook signature declared as optional?

We'll take a look at this issue during our Offsite, week of March 18th.

The relevant code was moved from ChangeTags into ChangeTagStore by @Ladsgroup in June 2023, see https://gerrit.wikimedia.org/r/c/mediawiki/core/+/926469

But the old code was just:

		$services = MediaWikiServices::getInstance();
		$userObj = $user ? $services->getUserFactory()->newFromUserIdentity( $user ) : null;
		( new HookRunner( $services->getHookContainer() ) )->onChangeTagsAfterUpdateTags(
			$tagsToAdd, $tagsToRemove, $prevTags, $rc_id, $rev_id, $log_id, $params, $rc, $userObj );

That would also indicate that the user would be null if not provided.

Change 1011383 had a related patch set uploaded (by Daniel Kinzler; author: Daniel Kinzler):

[mediawiki/extensions/EventBus@master] EXPERIMENT: Add fallback to onChangeTagsAfterUpdateTags

https://gerrit.wikimedia.org/r/1011383

I can't fiogure out what triggered the change in behavior. I made an experimental patch that may address the issue. However, I am not sure it is semantically correct in al lcases.