Steps to reproduce

1. On a content-app include a composite field
2. Create an item and check the lastModifiedDate
3. Edit the item and change some field within the compositeField
4. Check the lastModifiedDate is not updated
5. Change any other field outside the compositeField
6. Check the lastModifiedDate is updated

Expected results

The lastModifiedDate is updated

Actual results

The lastModifiedDate is not updated

Workaround

Do not use mgnl:content (but any type inherited from it instead) as node type of content items. 

Since the mgnl:content is used by default if not provided, therefore explicitly defined another nodeType under model of contentType would be a solution. see: https://docs.magnolia-cms.com/product-docs/6.2/Modules/List-of-modules/Content-Types-module/Content-type-definition/Content-type-Model-definition.html

your-content-type.yaml

datasource:
  ...

model:
  //any nodeType excepts mgnl:content, e.g: mgnl:contentNode, mgnl:category
  nodeType: mgnl:category
  ...

Development notes

N/A