Turns out the account associated with the vault had been denied "read" permission on the policy. Still investigating how this change came to be, but resetting the permissions appropriately resolved the problem.
Thanks to 'the green book'.
I have a policy in production that is not showing up in the "uncontrolled" tab in AGPM. I'm not entirely sure why. The policy was controlled at one point, but due to AGPM's refusal to import the most recent version from production, I deleted the policy from the AGPM repository and made sure to clean out the recycle bin afterward. Now the policy doesn't show up as uncontrolled, but is still very much alive in production (it should be). I can still edit the policy live, but I would prefer to manage it through AGPM like all my other policies. Anyone else seen any unusual behavior from the control/manage portion of AGPM?
Turns out the account associated with the vault had been denied "read" permission on the policy. Still investigating how this change came to be, but resetting the permissions appropriately resolved the problem.
Thanks to 'the green book'.