Can you show me the mail headers for the mail you received but did not expect to receive? In particular, the X-Phabricator-Mail-Tags header?

For example:

X-Phabricator-Cc: <PHID-USER-xoq4jkvovuuheffcigaz>
X-Phabricator-To: <PHID-USER-jl4arf2nmmwptf2nw23d>
X-Phabricator-Projects: <#release_6.4>, <#defect>, <#qa>, <#mtm>
X-Phabricator-Sent-This-Message: Yes
X-Phabricator-Mail-Tags: <maniphest-column>, <maniphest-other>

It looks like some other activity also occurred in the same transaction set. Does the email reveal what it is? That is, do the first two lines look like this?

epriestley moved this from column X to column Y.
epriestley did something else here.

Occasionally we have a bug with transaction handling where a transaction can add a mail tag but later not actually appear in the mail, so if the mail doesn't make it obvious what the other transaction is we might have to go fishing around.

You might also be able to figure out what the other transaction was by looking at the task itself: the second transaction may appear in the timeline next to the column move.

The email itself is isolated to the one action: movement on a workboard. There is a second e-mail however which refers to the closure of a task. The two are effectively grouped together and happened quickly, but two separate emails were sent. The other email's headers:

X-Phabricator-Cc: <PHID-USER-xoq4jkvovuuheffcigaz>
X-Phabricator-To: <PHID-USER-jl4arf2nmmwptf2nw23d>
X-Phabricator-Projects: <#release_6.4>, <#defect>, <#qa>, <#mtm>
X-Phabricator-Sent-This-Message: Yes
X-Phabricator-Mail-Tags: <maniphest-status>

I'm going to close this in favor of T10448, which discusses fixing "other" tags and generally improving how tags work.