This seems to be related to https://secure.phabricator.com/T10978
We update from master weekly.
We have Herald rules in place that add auditors to commits, for example:
Until now, some commits were affected by those 3 rules and therefore 3 different auditors were added.
After the update to latest version last week, the following happens:
As you can see, Herald indicates that it's adding the different auditors but only the last one is kept.
Here you can see the Herald transcript:
This is happening in every case where there should be more than one auditor.
Let me know if I can provide more details.