I noticed a Gerrit change that included in the commit message, "This probably fixes T125323", caused Phabricator to close the task automatically. Though the commit was not merged, Diffusion indicates it is accessible through refs/changes, as is possible when retrieving the change directly from Gerrit using git. (Interestingly, a "Still Importing..." message is shown in place of a diff, which may be a separate issue.)

Either Phabricator should only close the task once the commit is on an actual branch, or the autoclose feature should be disabled for repos set up with Gerrit code review.