As described in T33, Phabricator used to describe what are now "Parent Tasks" and "Subtasks" as "Blocks" and "Blocked By", and unlike Bugzilla, only allowed users to "Edit [the] 'Blocked By' Tasks" of a particular task, not the other way around. (Even worse, prior to rOPUP4987f33bbcd0b9f8, the function was called "Edit Blocking Tasks", which was less clear and possibly even ambiguous.)
The confusing and inadequate UI meant it was extremely common to enter the relations the wrong way around. So existing relations of this type, created after the switch to Phabricator, but before the recent Phabricator upgrade, should be reviewed for accuracy, and corrected where appropriate.