See PHI1553. "ManiphestTask" currently implements this policy hint:

public function describeAutomaticCapability($capability) {
  return pht('The owner of a task can always view and edit it.');
}

...but it isn't actually shown in the UI. This was likely a side-effect of D20165, which implemented a "PolicyCodex". When an object implements PolicyCodexInterface and the interface provides policy exceptions, the deprecated "describeAutomaticCapability()" method isn't invoked -- the assumption is that the Codex provides a complete description of exception rules.

This ("Codex" + "describeAutomaticCapability") could be made an error, but this will probably never happen again before the codebase fully moves away from "describeAutomaticCapability()".