This article discusses how to validate a field's presence on the instance when the following behavior is encountered:

• Field is no longer present on the form as expected
• Field does not appear in Configure Form Layout or Form Design
• Can not confirm any UI Scripts or UI Policies that affect this field.

In this case, the most probable cause is the field has been intervened with at the Dictionary level.  It may have been set to inactive on the instance. 

Navigate to the Dictionary record:

1. System Definition > Dictionary
2. Filter where table is name of the correct table in the hierarchy for which the field lives (ex: task)
3. Filter for column name or column label to find the field in question.

You should be able to resolve the issue by locating and ensuring active = true, or verifying any other anomaly on the dictionary record.