Question:

Does updating the value of a lookup field on a document affect the value of the Last Modified Date field on that document? 

Answer:

No. When the value of a lookup field is changed on a document, this is not considered a change on the document and as such, the document is not considered to be updated. This means that the Last Modified Date field is not updated on the affected document as a result of this change.

Related Documentation:

Vault Help Documentation: N/A