Why has a field disappeared from one or more document types in Vault?
Normally a field is hidden on a document because a Field Dependency is set up to hide it in specific circumstances. This can be checked by going to Admin -> Configuration -> Field Dependencies. Sometimes though when reviewing the Field Dependencies it is found that there is not one set to hide the field that is missing, or at least not in the circumstances involved.
When the field isn't set to be hidden, there will be a Field Dependency that is set to unhide the same field. This could be for the same document type or a different one. An opposite behavior control is set up for Field Dependencies when they are controlled by the Document Field or Document Type. Unhiding the field on a document type with a Field Dependency controlled by either of these hides it on other document types. *Working as designed*
If the field is set to hidden in the field dependencies, and should not be, removing the Field Dependency will correct this.
If the issue is that a dependency is set to unhide the field then any Field Dependencies set to unhide that field should be removed. Once this is done, the field should appear again. To continue to hide this field in certain circumstances on the document type or types it was previously unhidden on, it may be necessary to add or modify other Field Dependencies.
Vault Help Documentation: Managing Dependent Fields