HONG KONG/LONDON—Ratings agency Fitch downgraded property developers China Evergrande Group and Kaisa Group on Thursday, saying they had defaulted on offshore bonds, while a source said Kaisa had started to work on restructuring its $12 billion offshore debt. The downgrades to so-called “restricted default” status come even though Evergrande and Kaisa have not officially announced defaults that could result in drawn-out debt restructuring processes. The fate of Evergrande, which has more than $300 billion in liabilities, and other indebted Chinese property companies has gripped financial markets in recent months amid fears of knock-on effects around the world, although Beijing has repeatedly sought to reassure investors. In its note on Evergrande, Fitch said the developer did not respond to its request for confirmation on coupon payments worth $82.5 million that were due last month, with the 30-day grace period ending this week, and so assumed “they were not paid.” Evergrande and …