Revisions may be related to tasks with a DifferentialRevisionHasTaskEdgeType::EDGECONST edge, either via Edit Related Objects... → Edit Tasks in the web UI or by running arc diff on a branch named Txxx.... However, this relationship is not transferred to commits, so commits don't end up attached to tasks and don't show up in the (recently updated) "Related Changes" table.
For now, I plan to treat these relationships like we treat Ref Txxx in the commit message, and effectively copy the edge when we discover a commit. Copying edges isn't great, but "Ref" and "Fixes" sort of work like this already, and even if this is walked back later it isn't a big deal.
See also T3577 (which has some adjacent discussion) and T11934.