I think Phabricator just reflects the list of subscribers in a task when it is being edited. In your example above, if you had opened the edit window before the user unsubscribed, they will still appear in your edit window, and it will be subscribed back when you save it.

If this is what you mean, then this is a variant of the 'no mid-air crash' problem (T78236).

No, it's not what I mean. It's about the username being mentioned in the *text* description, not in the subscribers field.

I recall having a similar issue with a comment recently (i.e. not a task description)

Current status, quoting upstream comment https://secure.phabricator.com/T11035#180691 from June 2016:

• editing comments uses the correct rule (only new mentions count).
• editing other fields still uses the old/wrong rule (all mentions still count).

Fixing the "other fields" case is complicated, per above, and I plan to wait and see how https://secure.phabricator.com/T9789 and adjacent work develop before planning how to continue forward here.

Current status, quoting upstream comment https://secure.phabricator.com/T11035#215569 from March 2017:

This is no longer blocked on anything, I just need to develop a specific technical plan to resolve the "adjust" vs "expand" sequence inside transaction application so the "expand" code has access to the old text by the time it runs.

I would think that this is less important now that tasks have an explicit mute which overrides notifications regardless of subscribed status.

For the records, this created confusion again in T283983