Steps to reproduce:

• Edit Maniphest config custom-field-definitions to include *required* custom field, e.g.:

{
  "mycompany.estimated-hours": {
    "name": "Estimated Hours",
    "type": "int",
    "caption": "Estimated number of hours this will take.",
    "required": true
  }
}

• Create a task
• Create a comment on the task mentioning another task, e.g.:

Something T22

• Edit comment above (leaving the the T22), e.g.:

Something else T22

• Click Save Changes

Expected result:

Comment changes saved.

Actual result:

Visually nothing happens (edit modal dialog stays on screen).
Browser developer console shows error:

POST http://<phabricator>/transactions/edit/PHID-XACT-TASK-p7kt4lrplc4qg6q/ 500 (Internal Server Error)

And buried in response JSON/HTML there is a an error message:

Unhandled Exception ("PhabricatorApplicationTransactionValidationException")
Validation errors:
  - Estimated Hours is required.

This is also reproducible when comment contains other objects e.g. a diff D123.

So far to me it seems that Phabricator on comment edit is trying to save an edge which "edits" a the task and triggers validation error due to required field not being in comment transactions.

Version information:
phabricator 5e423c5fe021cd2e97adcf71c5faaba685471b06 (Tue, Mar 21)
arcanist 3b6b523c2b236e3724a1e115f126cb6fd05fa128 (Feb 18 2017)
phutil 91ab940c3979f7bd7a7c74099c46fc8884a08128 (Sat, Mar 18)