Instance is at master HEAD running a series of Git repositories self-hosted by Phabricator. Due to another issue (see T11180), executed bin/repository reparse --all REPO --message. Several lingering PhabricatorRepositoryGitCommitMessageParserWorker tasks cannot complete seemingly due to the commit being missing somehow. The Data element of the task contains the commit hash, and it is linked. Upon going into the commit some metadata is available such as the diff and tasks linked along with a summary but the message:

This commit no longer exists in the repository. It may have been part of a branch which was deleted.

is presented. The task never completes and will retry indefinitely.