The last updated date on the Projects entity only shows the last date on which the fields on the Projects entity itself were updated – it does not flow through from all the underlying entities (such as Issues, Comments, Health Indicators,etc.). The underlying entities each record their own last updated date per record.
If for example, you edit the Department field, which is a field on the Projects entity, PPO will record the last updated date on the Projects entity.
However, if you edit a Health Indicator, the last updated date on the Health Indicator will update but the last updated date on the project will remain unchanged.