As backstory: I accidentally added the subscriber PHID-USER-abcd to T1 on this install by calling maniphest.edit. I intended to edit T1 on my local install.
This edit is permitted for messy technical reasons, described in T13429. It's not valid, but it's hard to prevent.
The state we reach is also possible even if the edit is rejected (i.e., someone can go manually update the database).
Regardless of how we get into this state, the state (a non-user subscriber) breaks the UI on the task page when it attempts to test if the subscriber can see the task.
To prevent this, only claim that a Handle can have capabilities if the handle is complete. If the handle is incomplete (an invalid or restricted object), it either can't be meaningfully tested for capabilities or the viewer isn't allowed to know them.