The Project sub-object needs to have a 'Properties' page in Modern UX and the sub-object is not a Custom investment (using Investment Extension) and was originally created the sub-object a while back and MUX was not considered using it in the MUX at that time.
Release : 16.0.3
Clarity sub-objects and sub-sub-objects do not have a Blueprint 'Properties' page in Modern UX. The Details Fly-out is the only 'Properties' type of page available at this time for the sub-object in MUX, and unfortunately it does not have 'sections' either. Thus, without a link to open the sub-object instance in MUX you would need to configure/use the Details fly-out to see the sub-object instance details.
You can vote for the following Idea via the Clarity Product Enhancement process.
- Blueprints & Business Rules on Subobjects (D711) - Ability to have Blueprint capability for stock & custom Sub-objects of Investments including OOTB (task/staff). This is equivalent to Properties window in classic for Sub-objects so that user can click through and see only relevant information along with sections if they have large set of fields to go through. Also, ability to have conditional view of sections like business rules in Investment Blueprints, this is equal to conditional sub-pages in classic.
Workaround: Create a custom subobject (using the Investment Extension) which Modern will generate a Blueprint and 'Properties' page...