As far as I know this is a feature that currently works fine:
e.g. from the test system:
{"entities":{"Q2":{"pageid":5,"ns":120,"title":"Item:Q2","lastrevid":7,"modified":"2021-08-11T08:10:30Z","type":"item","id":"Q2","labels":{"en":{"language":"en","value":"foo cat"}},"descriptions":{},"aliases":{},"claims":{"P1":[{"mainsnak":{"snaktype":"value","property":"P1","hash":"874d19e6a8080831f530365565a57dc9dd3ba463","datavalue":{"value":"cat","type":"string"},"datatype":"string"},"type":"statement","id":"Q2$158446eb-4a66-c356-6a79-3cd1e66bf622","rank":"normal"}]},"sitelinks":{}}}}
however right now these pages cannot subsequently be re-rendered (which is to be handled in T285293).
Within this ticket, we want to write some test case (probably in selenium) to ensure that the ability to save statements with local properties continues.
BDD Scenario: On an item page in a Wikibase with Federated Properties enabled
GIVEN I am adding a statement to a local entity containing a local property
WHEN I click Save
THEN I can view valid entity JSON from Special:EntityData/Q1.json
Note: Look at extensions/Wikibase/repo/tests/phpunit/includes/FederatedProperties/Api/EditEntityTest.php